Check your Q6600 stepping, if it uses the G0 stepping, it should be able to reach up to 3.6GHz nicely with a Vcore between 1.44 and 1.52V (Depending of the motherboard) I used to have an QX6850 (Basically an unlocked Q6600 G0 stepping) overclocked to 3.60GHz and since I sold it on ebay for a nice price, I went for the Q9650 which I overclocked to 3.82GHz and I can't really notice much of a difference besides much less power consumption and heat dissipation (The Q6600 series are hot and power hungry when overclocked)
But I felt my system more responsive when doing intensive file transfers, multi tasking and media encoding all at the same time (Probably the larger, faster cache comes in hand here). But I think that doesn't worth the investment overall, just overclock your CPU (Just check your CPU stepping with CPU-Z or similar tools and pray that's not a B3 stepping CPU)