I got 4 cores to 3.7GHz without working too hard--pretty lucky on that core for only paying $99 for it with the free Microcenter mobo. The whole build in an old case for $250 (no new graphics).
If you're gonna ask the question, "Is it worth it to O.C.?" The answer will always be "Yes" from these forums if you can get a stable improved clock rate at a reasonable voltage and heat output. Even a 5% OC is worth it. I asked myself that question when I baked my GeForce 7950GT back to life--and now I haven't an OC'd undead 7950GT for gaming at work at lunch.