The best graphics card in terms of performance is the Radeon 4870 X2, but that being said most of the sim racing games should work decently with the 7600GT you've got.
Like others said already: it's possible that it's not your graphics card that causes the lock ups or stuttering. It's quite difficult to pinpoint exactly where the problem is located in your case, it could be varies thing:
- drivers: did you update some lately?
- memory: could explain the lockups, you might perform a memtest to check
- hardware issue (power supply, motherboard, ...)