A couple of questions about overcloack

1) no idea
2) no idea
3) voltage to the PCIe slots - may help in overclocking graphics cards (or anything else plugged into PCIe - physics/raid controller cards) effect - voids warranties
4) Northbridge voltage - will help in big overclocks, but increasing the voltage drastically increases heat - so you may need a better heatsink and maybe even a fan if this voltage gets increased much (or even if you push FSB high enough)
5) depends on the mobo - usually change the CPU settings to manual and it opens up the setting.

With a 6600 the multiplier is limited to 9 - all the reviews I've seen show very little differences (<1%) between FSB's at the same clock speed. So while you might get a higher FSB at a lower multiplier - if it doesn't let you reach a higher clock speed then it won't give a performance increase.

Maybe this will help with the #4, on the motherboard there are two chips that integrate everything. The northbridge is the one that controls the: CPU, ram, graphics slots.

The other is the southbridge and has the "slower" stuff like the SATA, PCIe x1, on-board network/audio/video, etc.

So when you overclock the amount of info going through the northbridge increases, and the same way as increasing the voltage on the CPU, if you up the NB Vcore it may help everything run better.

My point about the fan, the heatsink on most northbridges are passive and only meant to cool it at stock settings. As you increase voltage it *really* increases heat produced, so you need a fan or better heatsink on top of the northbridge to keep it cool. Which again is exactally like a CPU.