Agreed, the 512mb version is better, but in whole they are both crap cards. The 7900 is better than the 8500GT. What will you be using it for?
They're both crap, but the 512MB is better. If you want to play games on the system, think about getting a decent card like an X1950 Pro, 8600GTS or HD3850. Also consider that your PC will probably need a new power supply if you're upgrading the graphics card.