first things first....where do you see an fx-55 for under 100 dollars? the only one i find on new egg is an open box OEM for 178.
hmmmm...i'm torn.
the fx will have an unlocked multiplier. so it will (most) likely be easier to overclock than either the opty or the 4000+.
the 4000+ is a good chip. the fx is a fantastic chip, so is the opty.
i think this time i am going to have to disagree with my brethren and say that if you can get an FX 55 for 30.00 more than a 4000 (which is 59.99)..that is going to be the way to go as far as sheer gaming and overclocking performance. definitely, no doubt about it. the unlocked multi will help overclocking A LOT.
bear in mind...the opty is dual core, the fx 55 is not. if it is under 90-100 bucks and you are going to be like surfing the net and playing games constantly...dude there is no question, fx all the way.
edit: zmatt no doubt your opty is a kick a$$ chip. tho.