You've got DDR2. You need DDR3. The ones to get are the G.Skill Ripjaws 2x2 GB DDR3 1600 mhz CAS Latency 7
XCLIO's PSU aren't that good. I'd get an OCZ ModXStream Pro 700W. It's in a combo with the Samsung Spinpoint F3
for $95 after rebate.
I would highly recommend looking at some AMD builds, specifically using the Asus M4A79XTD EVO
($110 after rebate). Check out the X3 425
, X4 620
or higher. See if those would allow you to get an HD 5850.