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.