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I guess it depends on what you plan to run in Windows, and whether you plan to run it under virtualization or via boot camp.

In boot camp, I would suggest 64 bit since you are otherwise limited to 2GB of RAM.

If under virtualization, it probably depends on the amount of real ram you have on your Mac. 64 bit uses more RAM, so if you only have 4GB, you may not want to use up the RAM Windows would require.
 
Unless you can find the 64-bit drivers for the Mac hardware somewhere I'd say use the 32-bit. Unless MS has all the drivers. I haven't looked lately.

Drivers are on every Leopard and Snow Leopard DVD since 2008. The MP3,1 uses EFI64, so there is no problem whatsoever. Do not use setup but Bootcamp64.msi from the Drivers/Apple folder to install drivers and make sure you have admin rights and have UAC disabled. For Seven you must use the compatibility option to make the Vista drivers fit before you run Bootcamp64.msi. You right klick to do that.

XP64 is not supported but Vista64 and Seven64 are.
 
Drivers are on every Leopard and Snow Leopard DVD since 2008. The MP3,1 uses EFI64, so there is no problem whatsoever. Do not use setup but Bootcamp64.msi from the Drivers/Apple folder to install drivers and make sure you have admin rights and have UAC disabled. For Seven you must use the compatibility option to make the Vista drivers fit before you run Bootcamp64.msi. You right klick to do that.

XP64 is not supported but Vista64 and Seven64 are.

Sounds about right except Snow Leopard wasn't out in 2008 so there might not be some of the newer drivers on it for older hardware.
 
Sounds about right except Snow Leopard wasn't out in 2008 so there might not be some of the newer drivers on it for older hardware.

The Snow Leopard DVD has Bootcamp 3.0 version drivers. They definitely include 64bit drivers for the 2008 Mac Pro.

The 2008 Leopard disk that came with the Mac Pro has Bootcamp 2.0 version drivers. They should be fully functional in 64bit for the 2008 Mac Pro as well. This was the first Mac that came out with the EFI64 and did not have the annoying fault with booting MS 64bit DVDs.
 
Not an expert, but....

Advantage is mostly how much memory your applications and OS can use. Especially if you use Lightroom or Photoshop which are 64bit apps. PS CS4 is 64 bits on Windows but only 32bits on OS X for now.

Max memory for 32 bits on Windows is around 3GB even if you put in more memory unlike the 32bit OS X who's got a higher limit.

Definitely go with 64bits, I have it installed on a WIN 7 RC/Beta via Bootcamp before and after Snow Leopard without any problems with the drivers. I do remember updating the Audio driver (pre Snow Leopard) but it was rather painless. I'm using a 2.8 Early 2008 Octo.
 
Max memory for 32 bits on Windows is around 3GB even if you put in more memory unlike the 32bit OS X who's got a higher limit.

Have you actually booted a Mac Pro 2008 with 3GB or more of RAM in 32 bit Windows? When I've booted 32 bit Windows XP natively (boot camp), it only saw 2GB. I figure it had to do something with how the firmware allocates the memory map. Since Mac OSX boots a PAE kernel, it can see all the memory.
 
kevink2, are you seeing only 2GB on a 2008 mac pro? With 32 bit XP on my 2006 mac pro, windows only see's 2GB (1.99GB, actually... i think).
 
Win XP 32bit on a 2008 Mac Pro will only see 2GB of ram. 64 bit Windows doesn't have this limitation.
 
kevink2, are you seeing only 2GB on a 2008 mac pro? With 32 bit XP on my 2006 mac pro, windows only see's 2GB (1.99GB, actually... i think).

Yes, even when I only had 4 or 6GB of real RAM.

I rarely run Windows directly on my Mac, though. I generally only run Quicken on my Mac's copy of Windows, which is why I'm not in any rush to upgrade it to Windows 7 :)

In VMWare, 32 bit Windows supports the approximate 3GB that 32 bit Windows will normally support.

I generally do my Windows programs on my aging laptop. I don't know what I'll do when that dies. Go cheap/small (netbook?) Or nicer like a 13" Macbook?
 
I ended up doing 64 bit and it worked great till i installed bootcamp 3.0 drivers and was unable to use my apple blue tooth kb and mouse
 
I ended up doing 64 bit and it worked great till i installed bootcamp 3.0 drivers and was unable to use my apple blue tooth kb and mouse

I installed Windows 7 64bit on my 2008 Mac Pro on a designated hard drive. No Bootcamp and it worked perfect, 7 automatically installed drivers for everything, including my apple bt kb. I added Bootcamp 3.0 and had the same issues as you, it bricked the bt kb under Windows. Just do a system restore before you installed Bootcamp. This is all under the assumption you have installed it on a second hard disk of course.
 
Bootcamp drivers work if you install Windows with Bootcamp methods. It means you have the right partition map (GUID) and not MBR, you have a proper EFI or GPT protection partition and an unpartitioned 128 MB end zone. If you choose to disregard Apple's installation procedures any incompatibilities are on your own risk.
 
I have done a Bootcamp 3.0 on a MacPro4,1 today with XP32. I had to manually install the Intel chipset driver because I got tons of system hardware not recognized. I also had to install the Intel Ethernet drivers manually. Apologies for my previous post. There is certainly something screwy in Bootcamp 3.0 and XP. But although the process isn't fully automatic you do get all drivers from the Snow Leopard DVD. It just takes a bit of manual prodding.
 
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