Darn, that's too bad you can't get to 4GHz, but it isn't a special magical barrier, it is just another ~5% clock speed, which honestly wouldn't likely be very noticeable.
Sorry, I missed your earlier question... what resolution do you game at? You'll probably bottleneck the GTX680 a bit, I game on a 4.03GHz PhII x6 with a 7970 (pretty close in performance as a GTX680) and have no complaints, though I am sure the Thuban holds it back some. But I play more RPG's and strategy games, so it isn't really a problem for the games I play. The PhII is a good CPU, but it is what it is, a budget part these days... don't expect miracles.
What happens if you push the multiplier to 19.5, it crashes? I run my PhII at 1.5v, but that is probably pushing it for most people. Any luck unlocking it to an x6?