From the reviews that I've seen (mostly at Tom's Hardware) you're much better with the 4000+. This CPU sometimes manages to outperform the FX55, and it's much cheaper.
But how about a x2 3800+? It's a dual core CPU much cheaper than the FX55, but in many circumstances will beat it. It's more expensive than the FX4000+ but I think it's a worh price. Current games do not benefit that much from a dual core, but the OS will run much responsively and running multiple aplications will greatly improve. On the long run software will be adapted, so that means longer life
.