Um, well a tower's the same thing as the ones that you lay down, it's just a different case. You might be able to find a case like that, not sure, they've been phased out more or less. I know Dell still makes comps that use them, so I guess there's gotta be cases like that somewhere.
Anyway, I suggest using www.newegg.com
for everything. With a price limit of $1000 or so you can build a really nice comp.
1.)Get a nice motherboard, with Serial ATA, maybe with a PCI x16 slot or two. Make sure it supports the CPU.
2.)AMD processor. A 64-bit would be sweet, but if not it'll still kick ass. Great for gaming. Be sure to get a good heatsink. Thermaltake makes good stuff usually, and Cooler Master is pretty decent as well.
3.)Try Crucial, Kingston, or OCZ for RAM. Get the fast stuff, and get two 1GB sticks.
4.)Serial ATA hard drive. The faster the RPMs, the better. I've had good experience with Western Digital, so try checking them out.
5.)A DVD burner would be nice, along with a 2nd DVD-ROM drive (for burning copies of CDs/DVDs). For $8, you might as well throw in a floppy drive just for the hell of it.
6.)Get a nice case. Read reviews, you don't want a flimsy case. This is pretty much your preference. I personally like black towers with the acrylic windows and lights. They usually come with a case fan, and also a nice power supply. You'll want at least a 400 Watt PSU. Oh, and if you want more cooling buy a few case fans as well.
7.)An Operating System of some kind. WinXp is about $100, but of course there are other ways of getting it....
8.)A nice video card. I suggest ATI. You can also get a sound card if you want, but it's not really necessary.
That should just about do it, assuming you have a monitor, keyboard, mouse, speakers, etc.