Unless you are testing programs in Os's for end users, quite frankly, you would only use WinXP x32 & Windows7 x64
WindowsXP 32 is more likely to have everything work better than vista32, because it's old, reliable & companies are well versed in it's quirks.
Advantage of Vista32 is ... DX10 Video hardware support (for games), but I understand there is a hacked version of DX10 for WinXP anyway
Windows 7 is the latest 64b offering, which is a great improvement to Vista64. Quite frankly, you can do away with installing Vista (any version), unless you can find and extremely compelling reason for it (aka testing for end-users).
And if you are installing multiple OS's to a (striped) raid array, I'd recommend a seperate (5400/7200rpm) HDD to store important documents/backups.
So, now you're down to Window7 X64 & WinXP x32 ... which I believe you will find most knowledgeable people would recommend. WinXP32 for Work, Windows7 for (new) games. Windows 7 is also free atm .. for the beta, and I understand that there is a public Release Candidate to be released later this month.
If you want something that all programs work in, you'd need to install MacOS & Linux, but I doubt that's what you want.
The advantage of a 64b OS is that it has access to >4GB of addressable space. Any programs designed for WinXP (32b programs ... which is the Vast majority currently, and for the next few years ... won't significantly benefit from a 64b OS (Except Games & Corporate/Scientific software).
Currently, the only programs which will benefit from a 64b OS (for home users) are games, which chew up lots of Ram ... but even then, it's only a matter of accesible RAM, as most are still only running 32b instructions, but use a 64b addressable storage space.