the 5200+ doesn't exist on S939 platform, the highest end was one of these:
1) Athlon 64 X2 4800+
2) Athlon 64 FX-60
3) Opteron 185
EDIT: the current cost of one (ebay) is usually more than what you would spend on new mobo/cpu/ram set
ie S939 isn't worth upgrading unless you have a crappy single core and can get a dual core for <$50