940 vs 810, Clock rate isn't as important as the CPU's performance itself. It does affect it, but Intel has proven that Clock rate isn't everything. The i7 920 is only 2.66 but it kills the AMD 840.http://www.anandtech.com/bench/Product/80?vs=82
Ahhh I guess the 460 SE is better than the 450 GTS, but when the 460 SE loses to a 460 768mb of the same price. That's pretty sad.http://www.techpowerup.com/reviews/Gainward/GeForce_GTX_460_SE/7.html
As for the 6850, the extra money would probably benefit in more frames, probably allow for more graphic goodies (AA/AF etc. etc.) The 460 768mb would be fine as well.
If you look in my guide, I have a seperation between the 600$ and 550$ build. Instead of the 6850 you get the 460 768mb and Destroyer dropping your price down to 550$. Leaving you still with better performance and room for expandability.