Well, he wanted to upgrade the CPU, not the whole computer, so he needs something that is socket compatible with the FX-4100. FX-4100 is an AM3+ socket, so his choices are:
FX-6350 Six 'core'. FX-8320 Eight 'core'. FX-8350 Eight 'core'.
Or, you could just go big with the FX-9000 series - the only difference being the speed between the 8000 and 9000 series FX processors (except of course for power usage!), but at that point you're running into the max as far as your price range. The general consensus though is that the 6 core FX-6350 is the best compromise between power usage, cost, and capability depending on your usage scenario. Cost is about $130 USD for an almost 4ghz 6 core processor.