I would choose my desktop OS depending on my server OS to have the same version of IIS. If the server uses 2003, you can choose XP Windows XP x64. If it uses 2008, you should choose Vista.
Even if your server was running in 2003, you can choose Vista to explore the new features of IIS 7 and thus prepare for the day you decide to switch to 2008.
I really see no reason to run 32 bits unless you plan to use some component that does not support the 64-bit version (for example, WSE 2.0).
So Vista 64 bit is probably the best choice or XP Windows XP x64 if you are running Windows Server 2003 and want to play it safe.
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