lyka0913 :
FX 4100
MSI 880GMS-E41 (FX)
500 GB WD
8GB RAM 1333MHZ KINGSTON
650 WATTS PSU
Thermaltake v3 BLACK EDITION VL80001W2Z (case)
As is so often the case, the answer is...it depends. Without knowing what you are looking for in your upgrade or what your budget is, it's hard to say. Probably the best overall upgrade to that system for both general use and gaming would be to get an SSD. They're a lot more reasonable than they used to be and the performance keeps getting better and better. You can get a 120 GB Samsung EVO 850 for $75 at Newegg or Amazon, or the same thing at 250GB for double that. Tom's Hardware has an article "
Best SSDs for the money" that list those and more options. You should give it a read.
After that, if it's better gaming performance you're after, Tom's has a similar article for graphics cards
here. You can see where the GTX 650 sits in comparison to some other cards in the hierarchy chart and there are some good recommendations by price range.