What speed can I get with an Nforce750i and a Q6700?

Sigh.

If you have overclocked your FSB, you have overclocked your processor. Running the FSB higher is the only way to overclock a Core2 CPU.

System specs?

How fast you can go depends on a lot of factors: CPU VID (the lower the stock voltage the better), motherboard (needs to have a pretty adjustable BIOS), memory (the faster and the lower the voltage, the better), cooler (some are much better than others), and case (the better it can move air through itself, the better the system cooling).

And some people just have more experience.

There are a lot of guides out there. We call them guides for a reason. They are not cookbooks.

This should be your first stop.
HOWTO: Overclock C2Q (Quads) and C2D (Duals) - Guide v1.6.1
http://www.tomshardware.com/forum/240001-29-howto-overc...

Next stop should be a guide for your particular motherboard. Google is your friend.

Go through the guides. Then go into the BIOS and change the System Memory Multiplier (or whatever your BIOS calls it) from AUTO to 2.00, 2.00B, or 2.00D - whichever you need to set the Memory Frequency to twice the FSB. Then when you increase the FSB, the memory clock will rise in step with it. If you are at 3.0 GHz (333 MHz X 9), your memory clock should be at 667 MHz.

If you left your memory settings on AUTO (or whatever your motherboard calls it), there's a good chance that you are overclocking your memory and that is limiting your CPU overclock.

Overclocking RAM:
http://www.tomshardware.com/forum/251715-29-ratio-myth

Download CPU-Z to check your FSB:RAM ratio.

Warning - confusion factor between what the BIOS calls things and what CPUZ calls things. What the BIOS calls "memory frequency" is actually the memory clock. What CPUZ calls "memory frequency" is half the memory clock - DDR2 RAM, remember? It transfers two chunks of data each bus cycle. What you want in CPUZ is a 1:1 FSB:RAM ratio.

Go through the guides. Then go into the BIOS and change the System Memory Multiplier (or whatever your BIOS calls it) from AUTO to 2.00, 2.00B, or 2.00D - whichever you need to set the Memory Frequency to twice the FSB frequency. Then when you increase the FSB, the memory clock will rise in step with it. If you are at 3.0 GHz (333 MHz X 9), your memory clock should be at 667 MHz.

Thanks - Quite nicely answered and a lot of good data. Appreciate it all - Including the sigh.