If you want to keep the socket 939, get a AMD Opteron 185 Denmark 2.6GHz 2 x 1MB L2 Cache Socket 939 110W Dual-Core Processor if your mobo supports it. Very overclockable.
The GTX280 will be slightly bottlenecked with a e5200. You will be getting the maximum experience that your computer can deliver with a high end card.
e8400 would be better.
For RAM go with the qualified vendor list for your motherboard.
I have an e7300 OC at 3.3 Ghz , it benches slightly higher than a stock e8400 so cache isnt that important in the games Im playing . I havent tried to push it higher since its doing what I need it too , and it didnt need any extra voltage to hit that figure
Have a look at AM2 940 quad core systems , with a 790 series motherboard . These are good value and the gaming potential is very good . At least as good as an intel C2Q and cheaper system cost .
I had originally a 2mth old quad core athlon system (9550) 4 GB DDR2 800 Ram coupled to a ECS 512 Mb 9600GT video card and a decent Gigabyte AM2 GA - Ma770-SEP mobo , within the last two weeks built up an Intel Dual Core E7300 to compare after reading the 7300 could oc significantly and wanting the best bang for limited buck
Obviously the mobos are different The intel having a P5Q-SE
Benchmarking using same ram, hdd and video card - over the course of the same several games etc and real world BM's I gotta admit the E7300 oc'd (stable) at 3.35 is almost double the speed in real world Bench marks and eats the athlon in Crysis, Moh airborne COD 4 and WaW I only run at 1280 x 960 on a 22 inch LCD as poor eyesight means when I go back to normal desktop use text is sooooo darn tiny
I'd have gotten the 5200 as it ocs higher but no stores had stocks left
So in my own testing i'd have to sadly recommend the intel way, this coming froma long time Athlon supporter is hard I tell you but honest