There is a decent bit of performance difference between the FX-55 and 3800+. The FX-55 is faster in every way when it comes to gaming. Here are some relevant benchmarks.
http://www.anandtech.com/showdoc.aspx?i=2249&p=8
If it were me, I'd seriously consider the dual core processor over the two single core processors if you want to stay with AMD. The FX-55 is going for $279, the 3800 is going for $109 and the 4600 x2 is going for $235 so its a nice in between processor for the other two.