Right now I have everything working on the following hardware with OS X 10.6.5:

P6X58D-E
i7 920 D0
6GB Corsair RAM
ATI 5850

However, I can't boot into the 64-bit kernel if I have SleepEnabler.kext installed along with NullCPUPowerManagement.kext. I will list the combinations I've tried.

NullCPUPowerManagement.kext: 64-bit and 32-bit both work.
SleepEnabler.kext: 64-bit and 32-bit both work.
NullCPUPowerManagement + SleepEnabler.kext: Only 32-bit works.

So I now have to choose between performance (my 920 is overclocked) and sleep if I want 64-bit, or I can have both with 32-bit.

Does anyone have an idea as to how I may go about solving the problem?

Thanks in advance for any help.