Too bad you already bought the motherboard. Sandy/Ivy Bridge is the way to go.
However, since you do have the motherboard, I would say to go for the FX-6100. Though, PLAN ON OVERCLOCKING IT IF YOU WANT GOOD PERFORMANCE!!! Seriously! Its really easy to overclock, and you can hit 4.5Ghz or more very easily. If you do not want to overclock, I would suggest staying away from the FX line of CPUs. Look for a Phenom II x6 on ebay, or go with the x4 955. Hopefully, Piledriver will fix the woes of AMD and you can upgrade to the new generation when it comes out. Hope this helps!