well if you have a am3 board you cant even put a BD chip in unless you have a am3+ board. the fx 4000 should have more overclocking potential but it will be more expensive. I say either way you need to wait to see what the prices do on the 955. they could drop to $100 possibly since the fx4000 is going to be $120 just like the 955. I say wait for more benchmarks and price cuts then decide.
I pointed out that I already own an am3+ mb but without new bios which may be required for bulldozer to work tho I'm not 100% sure.
Waiting is probably a better idea but it's frustrating and I doubt price will change noticeably