I believe the best choice if you're looking at AMD right now is the FX8120, its unlocked and will overclock very nicely so long as you have a decent cooler. On the other hand, an Intel Core i5 2500K is a little faster in most things when benchmarked, and will also overclock very nicely. Once you go higher up the Intel range AMD really doesn't have anything that competes.
While AMD generally works out cheap for chip + mainboard, it's also important to remember that their current chips generate a fair bit of heat when overclocked so you have to factor in the price of a high end cooler like
this one. With a i5 2500K they run somewhat cooler and
this cooler would be sufficient for overclocking to the same performance levels as the overclocked AMD chip.
Basically, when comparing whats better value overclocked, you have to factor in CPU + mainboard + heatsink cost.