Yeah man, don't worry about it. Yes, AMD's are better for gaming, but the differences are not that big, and it just ends up coming down to the GPU more than anything else.
The 6xx series are very good OC'ers, however, the 945 chipset is not the best. But you should still be able to get 3.6-3.8 out of that 640 very easily.
If you could gather the change to through a 6800GT in there, you will be very happy with the gaming performance of that machine.
BE HEARD - Techonvent
DS3 | E6400 - 3.2GHz 24/7 | 2GB OCZ PLat. PC6400 | 6800GT | Zippy 460W
What the world needs is more geniuses with humility, there are so few of us left.