Suggestion for a good overclocking CPU?

I can spend round [200-300] $ for a CPU. Eventually I don't want it to be overclocked. I'll maybe change his clock once my computer won't be able to support current games on market.
In the case I find a cheap CPU with good overclock capacity only able to run (example) Crysis overclocked, can I keep it overclocked ONLY when I use Crysis and take it back to normal clock when I do simple task without restart the computer or whatever?

What about the i7 920 or the E8400. I've always been workin with intel CPU... For what I've heard, they have a better OC capacity even if the multiplier on AMD black edition are unlock (from what I know).