P8P67-M Pro motherboard seems to be a good cost-effective choice for this type of build. It offers both SLI and crossfire support (8x/8x) so you have options for future upgrades. Bad part about the P67 motherboards is it's going to be hard without you going with one of the extreme edition motherboards for you to be able to use your sound card. If you use two video cards on any of the more mainstream boards, you'll cover up the slots you'd use for your soundcard. On-board sound has really come a long way, though!
If your purpose is primarily gaming, the 2500k should suffice, but if you can get the 2600k, I say go for it.
That's a good power supply, though you might require an upgrade depending on which way you go with your video card selection. The 5850 spec states 18A are required on the 12V rail (216W at max load each) so you'd need 36A just for video if you crossfire and another 10-12A for the CPU and anything else pulling power from this 12V rail (Thats around 600W required on the just the 12V rail!).
Western digital black drives are my choice for HDD on any build. I like to use an SSD for my OS/boot drive.