an HP tank is a direct replacement for co2. so you don't even need a regulator. also a 68/45 will give you about the same amount of shots that you currently get out of your 20 oz co2 tank. anything smaller will not yeild as many shots per fill.
you can also get a "stubby tank" in aluminum. steel tanks are ussually cheaper but only go to 3000 psi and are not as recomeneded. the resale is horible.
The difference between 3000 and 4500 aluminum tanks is what is used to wrap them. Fiberglass like the ACI tanks will look white. Carbon fiber (grey or black) can hold more pressure.