btw, the IRQL error doesn't have to do with assigned IRQ's. This happens when a kernel mode process or driver attempts to access a memory address that it doesn't have access to. It's usually a driver issue, but when overclocking, kernal process errors (i.e. the guts of windows) are pretty common.
I have to say, 1680 is damn high. You usually only get that with watercooling. Anything in the high 1600's with air cooling is pretty rare, and if you are using air cooling your case probably sounds like a few vacuum cleaners. I assume you are using a Black label fan for this monster o/c?
You o/c by changing the multiplier first, then tweak the FSB. See how fast you go, then drop the multiplier down by .5 and see how far you can up the FSB. See which one is better (in actual benchmarks). Multiplier won't screw up your system cards or memory, but increasing FSB speeds up everything. A 1600 o/c with FSB will run faster overall than a 1600 you got from just multiplier o/cing. But you might have better benchmarks at say 1550 with FSB oc/ing than a 1600 you got with straight multipliers.
What is your fastest STABLE o/c? I mean by running stuff like Prime95 or 3dmark2001 at demo with high resolution for a few hours?