1) Why do you want to change from the fx4100? Is this just a true upgrade?
2) Are you planning on sticking with AMD? Most of the folks on this board will tell you to ditch the CPU and mainboard and just buy Intel.
3) What kind of performance do you need? Will you be playing games, transcoding video, or just surfing the web and reading email?
4) What kind of budget are you looking at?
If you want to stick with AMD, but are looking for beter performance than the 4100, then step up to the fx 8120, since it seems to be the price/performance sweet spot for the BD line.
Houndsteeth is right, if you really want to see any real world improvement you'll probably have to get at least an i5. Otherwise, if your comfortable with overclocking I would consider doing that as it could give you the performance bump you need