Both Vista 64 and Win7-64 run nicely on a MP2,1. I'm assuming the 2,1 refers to the 2007 3,0 GHz Clovertown Octad.
The EFI32 can make problems with some 64-bit distributions though. If the DVD contains anytime upgrade multiple versions the 32-bit bootloader can stall on those. In that case you may have to use a tool like vLite to single out an image. Downloaded versions of Win7-RC1 may not have that problem as far as I'm aware.