For OC, you should be able to get along with a 3-gas computer like the DR Nitek Trio or equivalent. This will allow you to do computer calculated diving with any Nitrox mix as back gas and 50% and 100% for deco. If you need tri-mix capability, you can always go to gauge mode and use your tri-mix tables or invest about 3x to 5x the money and buy a Trimix computer.
For CCR or SCR, you are going to have to shell out some big bucks on computing power, as detailed by others above.