I'm currently able to get a stable overclock of 3.8Ghz with my e6750 on my Asus P5N-D, but only when the included Northbridge fan is attached and let run at full speed. Which, believe me, is deafening.
I've heard that finding aftermarket coolers that fit these northbridges is quite difficult...
Since the awful incident of spilling beer through the top fan vents of my computer and shorting the bejeezus out of my faithful old 8800GTS 640 (and thankfully nothing else) I've been looking for replacements.
I've found a single GTX 280 or a pair of 9800GTS I could SLI at around the same...