you may have a 500W PSU but it only has 16A on the +12V rail... it's your choice, but with the new parts (if you even go with the suggested) you will need a better psu that can handle the power requirements... the watts are not as important as the amps.... IMPORTANT yes, but it's something like this that will make a BIG difference later on with stability.
also, the opteron I suggested has an L2 cache of 1MB vs the 512mb on the 4600+, besides the fact of the Am2 socket and the 939...
S939 is still a decent setup... just going to show it's age soon.
Dell 4K S2817Q / Logitech Z-680 / Func MS3 Mouse / Corsair K70
NZXT Source 530 / i7-3770K / Corsair H100 / ASRock Z77 Extreme4 / SoundBlaster Recon3d
Samsung 850 PRO 512GB SSD/Seagate 1TB/2TB/WD 2TB
4x4GB Corsair Vengeance 2133 / PNY GTX 980 ti/ Seasonic M12II 850W