It's very common for most OC'ers to go to about 3.4 ghz with a Q6600 before having to up the voltage. I have an EVGA 680i SLI motherboard with 4GB of SLI memory and a 9800 GX2. I'm overclocked with just air cooling and at max benchmarking, I never get over 54C at the core. I use RealTemp to monitor the heat of all 4 cores and its fine.
DuoOrb is my CPU cooler, with some artic silver compound. I also have 3 80mm case fans and one 250mm case fan not including the 2 fans on my power supply.
Just to be safe I did up the voltage to my Q6600 v0.125. Doesn't hurt it much. But just air cooling, I don' t know that you'd want to go much faster than this but the PC is smoking fast!!! Huge different over stock settings I can say that much. I get about 385 fps in Call of Duty 4.
Running a Mirrored Raid 1 set for my OS with 2 Seagate 7200.10 250GB SATA II drives and then RAID 5 for my data set where my games are loaded and stuff with 3 more Seagate 7200.10 250GB SATA II drives. These 3 together yield much better performance than what you'd get out of a WD Raptor drive. I push 150MBs, raptor gets about 95MBs sustained. I saved $70 and have twice the storage! Only thing that would improve the performance some, would be a hardware accelerated sata raid card with a PCIe slot. This would take the host cpu from making the hard drive calculations and allow a hardware card like Promise or LSI to do it. Onboard SATA II is limited in Vista to 135MBs on the bus itself. But if you go to a PCIe Raid Card, you can go up to 300+MBs. That's where you can move to like a 5 drive RAID 5 set and see some good performance from the drives!
But overclocking a Q6600 is simple. My benchmarks rate me higher than the Extreme Editions. But not higher than the new i7 Cores and that will always be that way because of the new DMA setup for Direct Memory access and the front bus saying goodbye. Amazing improvement there that overclocking can't compete with. =)