In one of the previous posts I questioned which Vista version should I install on my future computer with a x64 bit CPU - be it x86 or x64. I will go with x86 as x64 is not yet there where it should be. On the server front the situation might be different yet, there are products that just won't run on x64.
Top notch Team Foundation Server is in the group, unfortunately. Imagine buying a monster quad core x64 server with tons of RAM and then...you can't install TFS. It can be setup in dual server configuration but still, you have to run at least on x86 server (virtual computers might help, but they will eat your RAM and CPU cycles). Isn't that odd with such a perfect x64 candidate? And even the excuse of not having time before RTM doesn't work anymore as it is been quite a while since it was released to public.
It seems that even in server x64 world are difficult decisions (on what OS to install) to be made. I guess the bottom line is that you should install x64 OS only if you know exactly why it will be of use to you. Otherwise stick with prehistoric x86 architecture(and go ahead, blame Motorola, because they were late against 8086 - otherwise better architectured CPUs would rule the world, and we could have much stronger computers right now).