The DX10 feature in 8500GT and HD2600Pro is useless, it will barely run any DX10 game. It just like the DX9 features in Geforce FX is useless, the FX series can barely run any DX9 game..
So, it would better to get a faster DX9 video card than getting a low end DX10 video card that will barely run any DX10 game
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By the way, it is better to get one good video card, than dual video card. I'm pretty sure that one 8600GTS can beat dual 8500GT or HD2600Pro