To add to the Physx card, not being worth it....the 8800GTS you're getting, is already capable of calculating physics on it's own, because it has a built-in technique, used to render physics.
all the 8 series can render physics, well at least the 8600gts and the 8800 series. no insane, you got a nice build there. but if you want to spend a few more bucks go with a gtx
The P5N32-E SLI is a great board. Has alot of features, but also pricier than, the 650i competition. If you have the money, why not. But I would recommend the MSI P6N Platinum.