I have almost the same sys,you have but with a Nvida2 64 Mb and runs smooth.
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VISTA 64Bit
C2Q Q6600 2.40G with Tunic Tower 120
8800GTX ( Step up program )
GIGABYTE GA-P35-DS3R 775 with Crucial Ballistix 4GB
Acer 22" Widescreen LCD Monitor