The other guy is sooooooo wrong, i've played gta sa in the past with a pentium celeron 2.8 ghz, 256mb ram and geforce2 mx/mx 400 with 64mb vram
so, yes, you can play it at minimum and at 800 x600, that is a really shitty setup, so you should get better perfomance with that ati radeon 3100