This is an old thread I realize, but I have been playing around overclocking a Pentium D chip for a while now, I prefer to use this dinosaur for playing with OC's when the mood takes me because it's cheaper than an oil change to replace if I kill it. I'm adding to this just in case anyone else comes across this thread in hopes of an answer.
ANYWAYS, what I wanted to add in terms of the interfacing between a 32 bit chip and 64 os's, is there is a real communication issue when you go passed 60% OC, not sure why, probably an overloaded buffer or something, but I'm capable of OCing to 3750 Mhz in 32-bit win7, but I get a mem management BSOD 5 minutes into a P95 stress test if I go passed 3442 Mhz in 64-bit Win7. Yes, I've done multiple DIMM and Ram stick tests, no issue with the DIMMs or the Ram that I can find. (I chased this problem around for almost a year before resorting to a 32-bit OS)