edit: stay away from the 1,1 and 2,1 machines as the boot system (EFI) is 32 bit and they are not able to run the latest OSX. You are not limited to Mac operating system tho, Windoze or Linux work fine with these great machines
There's a way around anything...
I'm running a Mac Pro 1,1 with the 2,1 firmware and the Chameleon hack to get disk based emulated 64bit EFI on Mountain Lion. Rock solid with no issues and much easier to do than most people think.