evga 7900gs is a rippin card. i can play anything i try on super high/high settings.
you will not be disappointed with it. slap it in that 4200 and watch it fly.
i think that 7900gs is better. if it was a 7600gt, it would be closer. check and see if they sell evga within your budget.
i don't think a 7600gs really can use 512mb of memory.