basically, FX's are like Ferraris. They are nice, but if you have to ask how much they cost, then they cost too much for you. The price/performance ratio is pretty much the lowest you can find, unless you look at the P4EE, which is slower and more expensive. FX-55 is for people who 1) have a 1.23 zillion dollar allowance from their parents and are TRYING to find a way to spend all their money or 2) need to buy an overpriced chip because they think it will add 3 inches to their e-penis and impress their techie friends. If neither of these applies to you, then you should probably look elsewhere. In terms of out of the box performance, I'd go with like a 3200 or 3500 winchester, or, if possible, venice. for OCing, it's hard to beat the 3000+ and 3200+. 3800+, 4000+, and FX-55 (130nm versions) OC pretty poorly and their performance just doesn't justify their price tag.
As for the OCing, I don't understand why FXs are so desireable. Sure they have unlocked multis, but they are pretty much stretching the limit of the process they are made on. FX-55s usually only hit 2.8GHz on air, a 200MHz OC, which means you get 7% extra performance. My 3000+ OCs 750MHz, a 41% improvement. Some even hit 2.7GHz, a 50% boost. Venice can apparently hit 2.8GHz with no vcore adjustments, and 3GHz with a vcore boost, meaning it's both a better value and a higher maximum speed than the FX.