64 bit windows on Core duo 2?

x86-64 bit processors run 32-bit code just fine.

While it is largely true that 32-bit programs will run fine in the 64-bit version of Windows, this is not univerally true. Some times the drivers for particular pieces of hardware that interface to a particular program, have not yet been implemented. My recommendation would be to NOT use the 64-bit version of Windows at this time unless you specifically know that all of your Windows applications are actually supported in the 64-bit version. The software developer shold be able to give you that information.

Also many drivers from the Mac Windows drivers CD that Bootcamp burns for you won't work on the 64 bit version.

The 64 bit version of Windows would run, but in reality the 32 bit version of Windows is much more reliable, and would also run just fine.