A reasonably priced choice to the FX-4100 is the PhenomII x4 960T ( just got one of these and it's been working ok )
But you might want to also look into Intel based solutions with i3 2120, as it would be better for gaming and generally perform better than the FX.
The 960T is a Black Edition CPU with unlocked multipler, and your Mobo may be able to unlock extra cores from the CPU.
Mine did so it became a 6 core
But if you do that and overclock you might want to consider picking up an aftermarket CPU cooler such as Cooler Master Hyper 212+. (~30€)
Well if it fits your case anyway. The cooler that comes with the 960T is a bit flimsy, although it will do good enough job at stock settings.