Although not supported by Apple's latest Boot Camp software (v3.0+), it is entirely possible to run 64 bit Windows 7 on an original MacPro (MacPro1,1). I'm running a MacPro1,1 with two 2.66GHz dual-core Intel Xeons, 8GBs of RAM, four 250GB SATA drives, a SuperDrive, and an SATA Blu-ray/HD-DVD drive. In addition to running Snow Leopard, I want to run the 64 bit version of Windows 7 -- rather than the 32 bit version -- so that I can take advantage of all 8GBs of RAM. Enabling access to the SATA Blu-ray/HD-DVD drive and the three additional, internal SATA hard drives is a challenge under any version of Windows on a MacPro. There are a few tricks you need to know to make everything work, but you're going to run Windows on a Mac -- why should extra work be a surprise?
This post isn't intended to be a step-by-step tutorial. Instead, I'm writing it to offer the necessary hints for a successful build. Knowing that it is possible is more than half the battle.
First, the 64 bit Windows 7 installer DVD will not boot on a MacPro1,1. You'll need to create a new installer that will boot on a MacPro1,1. See: Jowie's Blog post for details on how to do this. The new installer will work as you expect a modern Windows operating system installer should work. Difficulty Rating: 3 of 10.
Second, Apple's Boot Camp 3.0+ installer for Windows will not install its drivers on a MacPro1,1. Again, I'll spare you the various technical and conspiratorial reasons given for why Apple doesn't support 64 bit Windows 7 on a MacPro1,1. Suffice to say that it's still possible to install the drivers without very much difficulty. See: John Robbins' Blog post for details on how to do this. Difficulty Rating: 1 of 10.
Third, with the 64 bit version of Windows 7 and Apple's 64 bit Boot Camp drivers installed, you may start to experience the Windows Blue Screen of Death (a.k.a., BSOD). I haven't run across a technical explanation for why this occurs, but I know how to remedy it (as long as you're willing to give up access to your Macintosh partition[s] when booted into Windows). All you have to do is rename C:/Windows/System32/drivers/AppleHFS.sys and C:/Windows/System32/drivers/AppleMNT.sys. I do this by adding "-rm" after the name and before the dot-extension (i.e., "AppleHFS-rm.sys" and "AppleMNT-rm.sys"). Doing this will keep them from loading after your next reboot -- which you will want to do immediately. Keep in mind that each update to the Boot Camp software (i.e., v3.0 to v3.1) will likely undo that renaming. You'll need to remember to go back and rename those two files if you want to avoid the return of BSODs after updates. Difficulty Rating: 1 of 10.
At this point you should have a perfectly functional MacPro running both Snow Leopard and the 64 bit version of Windows 7. You should be happy with this outcome -- there are plenty who will tell you that it isn't even possible. You probably should be satisfied that you got this far. The next step would be labeled "Here There Be Monsters" if it were on an ancient map. Be warned. I think I spent three full days getting it to work. It WILL work but you might just end up starting over from scratch several times if one of the steps goes wrong. So, why would anybody want to tackle the next step? Because they bought an SATA Blu-ray/HD-DVD drive that works under Snow Leopard but can't be seen by Windows. Because they have additional, internal SATA HDDs that work under Snow Leopard but can't be seen by Windows. The cause for both is the same: lack of support for Advanced Host Controller Interface (AHCI) when booting into Windows. So, the final step in my checklist is to enable AHCI support. We'll pick up where we left off:
Fourth, your MacPro defaults to a legacy controller mode when installing/booting Windows. Not only does this result in your drive controller running at 100MB/s (instead of its 1.5GB/s capability), it also keeps Windows from seeing any devices connected to the other five on-board SATA ports. After trying several different step-by-step instructions without success, I finally came across the instructions posted on the MacRumors Forums. All you'll give up after successfully completing this final step is the use of the Boot Camp Control Panel under Windows. Booting into Snow Leopard will require rebooting your MacPro while holding down the option key so that you can select the boot OS. Difficulty Rating: 8 of 10.
That's it! You've tricked out your first generation MacPro with Snow Leopard and 64 bit Windows 7.
 I'm not kidding about this. By "scratch" I mean all the way back to wiping the boot drive and installing Snow Leopard.
Great work! I would love to test that out myself but lack the hardware. Still this is the type of instructions I like to keep around even if I don't need it right now, some one else may be asking me about it.
fj100: I disagree. Jowie's instructions have worked for me multiple times. In fact, that step is one of the easier steps in the process (which is why I gave it a score of 3 out of 10 for difficultly). I'm sorry that it didn't work for you.