Some of them can. IMO it isn't worth rolling the dice at this point. The price difference between a phenom II x3 and an x4 (like the 955 BE) is small. It's even smaller if you get an Athlon II x4.
Know that it depends on your motherboard as well - so it may not be possible with your current system. I want to point out that, in most cases, you will not see much of a performance difference between an x3 and an x4.