My brother gave me his old 7800GT and COD4. The problem is that I need to switch motherboards since I don't have PCI-E. I figure I'd might as well use the opportunity to upgrade. So, I'm looking for a CPU and motherboard combo. Any recommendations considering the following?

-I want to play COD4
-I'm probably going to switch to Vista Home Premium or Ultimate (OEM)
-Already purchased 2gig of 800MHz RAM (OCZ gold, can return)
-Already purchased 320GB hard drive with 16mb cache (can return)
-The rest of the computer's use will just be applications (Excel, Word, Photoshop, Illustrator, etc) and usual browsing and playing various media.

Disclaimer, I know almost nothing about computers. Educate me and recommend products por favor. h: Thanks.