The Sandy Bridge chips are phenomenal and do excel over the current line of Bulldozer/FX chips by a good margin in most applications. Now personally if all you really want to do if game and that's your main priority then it's a lot easier and cost effective to just move up to an FX 8120/8150, but granted if you're looking for a performance leap, you really aren't going to find it in the extra cores of the FX series over your current FX 4100. And the FX 81x0 chips do come on par with SB in certain software, but overall SB still overtakes the FX series by a good amount depending on the software being ran of course.
I'd recommend upgrading over to the 2500K over an FX series chip if you have the money, or you could always wait and save for Ivy Bridge. Or you could always keep your current board and chip until AMD Piledriver, which is actually what i'm doing, as the FX's disappointing performance leap over my current Phenom II X6 made me just decide to keep my 990FX board and wait till the their next chips as it's the most cost effective thing for me to do.