I don't think there is a 3.0 ROM for the A2000 (same as the A500) so just go for the 3.1. The 3.1 kills a few bugs that 3.0 had anyway - it's a better setup by far.
I have an A2000 with very similar specs, except that it runs OS 3.5 and larger hard drives. I use that for graphics work, DTP, and of course the occasional game.
BTW - do not loose that dongle in the joystick port. It may be needed for software already on your hard drive!