A 7600GS has much higher vertex nd pixel shader units than 7300GT, so can handle heavy effects more easily than a 7300GT. A 7600GS has the same core as 7600GT(560/1400 Mhz) but clocked at 400/800Mhz.
perfomance wise
7600GS>6600GT>7300GT.
So get a 7600GS model with a good heatsink nd a fan for further overclocking in future.