I'm building a rig for diablo 3 too, making it better than i need for future proofing. The coolmaster psu tend to be a bit hit and miss, Id suggest getting a crosair...antec...a good brand. Stick with the 2500k or 2550k (depends on prices in your country) for future proofing. I was considering the 6850 too, but getting a 7850/560ti 448 or equivilent depending on prices; again for future proofing. So change the psu and if you upgrade your gpu it would be money well spent
I am running Diablo 3 beta. Your build will work great with this game. Personally I like EVGA Nvidia cards. My PC has a GTX 460 and I run the beta at full graphics on a 28" monitor with great results. You can get that card at newegg for $125.