I have been using FX-4100 for a while now, and I'm a bit disappointed by the way it is being handled on Asus M5A78L-M LX motherboard. First off all CPU is not getting a 100% load on prime95 and is dropping to down 3.3Ghz or sometimes even 1.4Ghz but for a split second. I have read somewhere that is because the MB is designed to do that to protect itself from overheating. Corsair Vengeance 2x4GB 1600Mhz RAM is not even close in performance compared to my workhorse i7 which has got the same ram, event though I have set it to work on 800Mhz in BIOS. The last issue I have got is overclocking. I can only get it stable at 4.3Ghz but I know some people are able to go over 4.5 without a problem.
What is the better option for me at this point, swapping FX to Phenom II X4 or grab a 870 chipset motherboard keeping the FX with me. Is the current MB capable of taking full advantage of Phenom's? I know that 990 chipset motherboards are now miles ahead of handling my current CPU but with limited budget there are not many options to go for. I also know that first FX series processors were a disappointment, having said that I am quite pleased performance/price wise.
Details of my build
Asus M5A78L-M LX
Corsair Vengeance 2x4 Gb 1600Mhz
Get a 965be cheap and can run bf3 ultra 40+fps on caspian boarder, and for your overclock up your voltage for stability brother fx-4xxx series is actually a dual core but lets not get into that a FX-63xx may have made you happier.
Phenoms are more reliable ad im very happy with mine and i ran the full ATx version of your board and got it to 4.1 ghz very stable by cranking the VCore up.
But then again if I was to shell out for a new CPU right now I would future proof a bit with a MSI 970A-G46 + a FX8350 if you wanted to stick with amd that is.