Valves
This part helps give higher flow to your marker. Since higher the flow of air, smoother operation of your marker. There's quite a few out there. Let's go throught few of them for you.
AKA tornado valve = They're made by AKA. This is the best and highest flow valve on the market. And it's the most expensive valve. I would suggest this if you have money to splurge. AKA stands for quality.
Maddman Rocket Valves = This is by far one of the best for the value and performance. This is valve is made to work on "low pressure" operation. So I suggest getting a regulator before you buy this valve since it requires the marker at 600 psi or lower. If you don't you could damage the valve.
*Note*
There is a repair kit that you could buy for rocket valve
NDZ = This is a good company that comes out with quality products. They have a high flowing valve with nice pin to boot. This is little bit cheaper than the maddman Rocket Valve and I believe it does the same job.
There's also other valves out there but they don't perform any close to these 3 at the top. I will just list the ones out there if you're on a budget and need a high flow valve. There are made by 32*, TASO, and Omega valve (Omega valve is sorta interesting valve cause it's made of Ti and has an oring on the valve screw). These three are made in same design so get anyone of them if you like.
*Note*
Lot of people seem to have leak problem with there valves. So try using the lapco cup seal. it's a cheap upgrade but it usually solves leak problems. Stock cup seals seem to get damaged if you tamper around with it. You can't see the damage until you really inspect it really really close.
Striker/Hammer
There's just few out there. This part is basically just to hit the pin with less mass. In doing so less kick everytime you pull and you get better accuracy.
DHC = This striker is made of Titanium. This is 1/3 less than your stock striker. There is a new version to improve on the wear issue.
Night Stalker = This is a new striker introduced by Insane Pain sports. There are 2 versions regular and XL version. XL is 11 grams lighter than the regular. This his made out of titanium at all metel to metal contacts and the rest is made of Nylatron. Then coated in brine for wear issue.(I would recommend this over the DHC)
Check it Products = They announced that they're coming out with it a year or so ago but still haven't released it. So keep an eyes out for this.
ACP = They're made of aluminum/zinc composite where the striker hits the pin and the rest is made of nylon. This came out while ago but had some wear issue. They're at retesting mode. This would be the best striker out there since it's 2/3 less than your stock strikers. But who knows when this will be re-released.
Mod Striker = This mod is tricky since you need you're trying to grind away tempered steel. You're going to need a diamond wheel cutter if you're using hand tool or a table gringer. If you don't have a steady hands you're pretty much asking for a trip to the emergency room... So think before you really want to try this mod.
here's a link how it should look like. Scroll down http://www.pbnation.com/showthread.p...hreadid=608457
There's few people that's thinking of coming out with delrin and other composite strikers but no news yet when they will be coming out so we'll see what kind of different strikers will be in the future.
Regulator
This part is sorta like the heart of your marker. Many people has asked me what is a regulator. No joke. Well a regulator is.... well it regulates the air in your marker. I would say this is the most important part of your whole marker so don't go too cheap on it. There's many regulators out there that work great with CO2 or N2/CA. Let's see what kinds of regulators are out there.
Palmer Stabilizer = This is made by palmer pursuit. This is the all time great regulator. Works like a tank and regulates both CO2's and N2's. This isn't cheap regulator so think about it before buying. If you do get this one... you won't regret it.
ANS = ANS works decent with N2's but lousy with CO2's. I've heard people having some functions problems with this reg. I've used this and worked fine.
Air America = AA makes 2 versions. The Viligante and the Messiah. Both are pretty well built regulators. I've used the Viligante and worked great but like the most of the regulators out there it doesn't work with CO2's well.
WGP = They have well built regs that works with CO2's and N2's like the palmer's but the ERGO the cheaper version is capped at 600psi. so if you don't already have a turbo valve in your marker. Your marker will starve for air and not function.
*NOTE* The ones I've listed above is the popular ones. There's lot more regulators out there. Go check with your local pb stores and check them out. Also above are the ones I recommend over others that are out there.
Expansion Chamber
Lots of people will tell you to not to get a X-chamber. It's not useless as people say as it is. The function of an expansion chamber is to allow the CO2 gas to expand from liquid to gas before being discharged out of the marker. Well because an expansion chamber is nothing but a tube with multi-chambers it won't keep the air consistant when air is discharged out. That's the difference between a regulator and a X-chamber. There's many staged X-chambers out there. Stages basically means how many chambers there are for the gas to expand. Since all expansion chambers work pretty much the same I'll just list the name of the companies out there.
ACI, Lapco, System-X, PMI, AVI, JAVA,
Vertical Adapter
This upgrade is mainly for compact bodies. This upgrade just allows more air flow into the LPC then into the valve. There are very few companies that make this piece. So let's see what's out there.
CIP Assult block = This is currently top seller for this upgrade. This has 15* angled for comfort and looks. It's all about comfort so if you don't like your gas thru or reg being in a 15* angle don't get it.
Mod Stock VA = You could also mod your stock valve for better air flow.
If you don't know what you're doing DON'T DO IT!!! this mod is bit harder then other mods. Since your have to drill a bigger hole close to the threads inside you could screw it up easily. So if you want to know how to do it go this this link.
www.ottersccustoms.com
Well I guess that's it. I did a search for other aftermarket VA's for compact that I remember but not anymore. It seems like CIP is the only surviving aftermarket VA out there. If you don't to go the poor man way just mod your own stock VA.
Bolt
Okay, this is IMO the last upgrade you should get. Your stock without the venturi (star looking thing inside) taken out. It'll work pretty well. There's so many different bolts out there. I'm having hard time keeping up. It was so simple just 3 years ago... Where as the time gone

Well, let's check out some of the popular ones out there.
AKA Lightning Bolt = This is made by AKA famous for there valve and VLM bodies. These are the highest flowing bolts you could get. It's not cheap either. I would recommend this bolt over others even the delrin bolts out there. Get this if you have the money
ACP Blue bolt = This is a new product that ACP just came out with. It's made of nylatron. It has been through some test and it doesn't swell even when the bolt is submerged in water for 2 days. (that's a good thing).
Bandit Bolt = These bolts are made by Bandit. These bolts are made of nylon compound. So you don't need to oil these since it's self lubricating. It's farely cheap and no more O-ring issue
Kingman ACS = this is new bolt by Kingman product. The just introduced it with there new product line just recently. This bolt is to stop balls from chopping when the balls don't go all the way down into the chamber. If you want a system to stop chops this is one path to go.
Jam Bolt = This bolt is a Anti-chop system bolt. it works fairly well. It works pretty much like the Kingman ACS but Jam bolt came out with there bolt first. Let them settle it in court or whatever. But anyways, This is a nice bolt to get if you don't want to spend for Eframe with ACE system.
Shocktech Super Fly = This is produced by shocktech. More and more people seems to be getting this bolt. I've used this bolt and it blows IMO. I had some wear issue with the metal parts of the bolt. It's just one opinion so you should try it yourself and come to your own conclusion.
Spudnuk'l Bolt = This is Spudnuk'l Paintball Products. This is one of the top sellers in the bolt upgrade. This bolt is great for high flowing markers. it also have O-ring slits even though it's made of delrin/nylon composite. the design makes it for great upgrade if you're going for Low pressure markers. I would recommend this if you're going for nylon/delrin bolts
Home MOD bolt = This is the cheapest way to get a high flow bolt. You'll have to take out the venturi out ( the star looking thing inside the bolt) then polish it. If you want to see the steps "how to" go to this link
www.ottersccustoms.com
There's more and more different bolts coming out every so often so keep your eyes out. Even ACP is middle of designing there own bolt also. Suppose to be compareable to the AKA lightning bolt from what they told me. So let's see what we get in the future.