Atari software and Amiga software are incompatible, so to run Atari software you'll need an Atari machine. A500s are more compact, but A2000s are a better machine for expandability. They're both the same generation so they will generally run the same software. Rom versions are important, too as some software will only run on 1.2 or 1.3 roms. 2.04 roms are better and the latest is 3.1 roms which will run the latest OS version 3.9 (providing you have an accelerator) The accelerators are easier to fit into an A2000, but some are available for the A500 as well.
These machines will _not_ run AGA games. It is possible to get graphics cards for the A2/3/4000 models to run VGA resolution games, most of which also require a fast accelerator. AGA games will only work on A1200 and A4000 machines. Degrader software is available to give backwards compatibility for games. Amigas have always been amazingly expandable (with the exception of A600s and CDTVs) with A1200s towerized running PPC processors and A2000s running 68060 processors.
Don't know if you're in Europe or North America or elsewhere, but certain models are more common in one area than another. Me I like my A3000T with Cyberstorm PPC and 060 processor with a Cybervision graphics card. Still won't run AGA software though :-(
Overall, if I were you I'd be tempted to go for the A1200 with a 3.1 rom and at least an 030 processor if you're looking to the best of Amiga's past and an A4000 or A3000 if you're looking to upgrade to Amiga's future. Hang around Amiga Org and you'll get an idea where we're heading. 8-)
