Heres a little point for you...
Some guys will call it Nitrous, as in NitrousOxide. That's just wrong. The tanks are filled with compressed AIR (HPA = high pressure air) or compressed Nitrogen (N2). Some guys get freaked out if you call it Nitro.
Anyway, REIFIDOM explained psi and C.I. to you. What you need to do is check out some Paintball sales web sites and look at the tanks. The tank descriptions will tell you specifically what the size of the tank in cubic inches and the pressure the tank will hold in psi. The higher the CI and higher the PSI the higher the price. Also, they will state if the tank is preset or adjustable, meaning it will have a preset psi regulation at the tank (the pressure from the tank will be preset to some psi probably 700 to 800 psi). An adjustable psi tank will have a regulator that you can use to adjust the pressure whatever you select. Adjustable is higher priced.
HPA will not suffer from "shoot down" (the loss of velocity from rapid firing). HPA will not cause your gun to freeze or give off that muzzle smoke that CO2 does. HPA tanks come with a gauge that will show what your current tank pressure is. A quick glance at the gauge and you'll know how close to empty you are.
HPA tanks are pretty expensive but they have alot of benefits. I have a 68 C.I. 4500 psi tank and it works great for me. If you have the spare cash I'd recommend picking up a HPA tank.
Edited by Crusader