CPU may be a touch warm but not to the point of alarm.
Voltages are good as expected.
It did this on sp2 before you loaded sp3 correct?
First drop down to 1 stick of ram and try a game if it's still the same then
Try running MemTest from a bootable cd on 1 stick at a time for at least 10 passes >
http://www.memtest.org/#downiso