I game and the Celeron is not too bad. And yes they do oc nice. I have mine at 2.98 ghz from 2.66 with a 1 degree celsius change in temps. I would go higher but mobo wont allow.
Celeron D is the EXACT same thing as a Pentium 4. The EXACT same thing. They use the same core, run at relatively the same frequencies, only difference is cache size. And even then it's very noticeable. In most benchmarks a Celeron D can out perform a Pentium 4, if not tie it. Don't just assume because the Pentium 4 is higher priced and marketed heavier that it's a better CPU. When in fact, it's not much better at all. Budget-wise, Celeron D is a better CPU.