I'm having a hard time pulling the trigger on an order and need your guys help... Is there much difference from a stock iMac with Nvidia GT 130 vs ATI HD 4850 for gaming? It would be nice to be able just to buy a stock machines rather than a custom with the ATI for $50. My real concern is overall gaming proformance and heat produced by the two cards. Current iMac has the ATI 2600 HD Pro, which I think is pretty slow compared. Thanks!