XP64 is very solid. Ditto for Win7 beta (but f0dder's beta caveat applies). Don't have enough experience using Vista so I can't comment on that.
IMHO: Using 64-bit Linux for anything other than a very specialized high-performance singleton
server doesn't make much sense. And to really gain the benefit, you'd also need to code your application up in native 64-bit, along with whatever libraries it would call.
Still, I guess that's the price you pay when you're responsible for issuing the Launch Codes.
But even then, I'd be more inclined to go with a 32-bit cluster or distributed solution if at all possible. So unless you have a very specific native 64-bit application you want to run, I'd stick to 32-bit for a Linux desktop.
The problem with 64-bit Linux is that it is not
full 64-bit binary from front to back
. Most of the applications, and virtually all of the libraries included in a "64-bit" distro are still 32-bit. So even though you have a native 64 kernal, most of the code that gets called isn't. And once you go that hybrid route, performance goes right out the window.
BTW: What are you running that needs so much RAM? (Are you responsible for issuing Launch Codes?
Sidenote: here's a good website for 64-bit info and news: www.start64.com
Check it out!
------<edit -removed an erroneous comment>