Both the Phenom 2 x6 and E6600 are 64bit CPUs. If you had something (ram or board you said) die, that has nothing to do with your CPU not being able to run 64bit code. If you switch from an AMD board with an AMD chipset to an Intel board with a different chipset, it won't be able to load windows because the harddrive driver is attempting to use the old AMD driver. They are not interchangeable. You said you installed win7 64bit on a different partition, so obviously the E6600 is 64bit compatible? (unless you meant to say you installed win7 32bit.) All of the Athlon 64 and many of the P4s were 64bit. I'm almost tempted to say all S775 Intel CPUs have been 64bit, but I think there were a few Celerys that weren't.
I had no idea this processor was 64bit. It's a T6600. I'm gonna give you the best answer because you took this thread much farther than I expected. You have to be right about the Intel vs AMD. I've swapped systems on Windows before but I don't think I ever went from AMD to Intel.