A single core of a Phenom II X6 runs it maxed out @1680x1050 at about 35FPS. Performance per clock doesn't matter much in a multi-threaded game, as the biggest bottleneck would be the lack of threads/cores. On a Pentium 4 you'd be looking at about 24FPS average. I never said PhII = P4 anyway, all I said was P4 could max the game, and that proves it.
I use a Sempron email@example.comGHz at work, and even gaming on it, found barely any difference in performance between it and my Athlon @2.5GHz (2-3 frames) in games like Mafia II.
Plus OP never stated his resolution for gaming, and at 1280x1024, even a stock Pentium 4 would max it at playable frames, so what I said before still goes.