Just go for Vista; WinXP x64 is pretty much useless to 99.9% of people; it is not worth getting if you haven't already got it. Just go with an Vista x64 install; Vista x64 drivers will be much more common then XP x64 drivers.
I wouldn't reccomend moving to Vista right now:
-it will demand more resourced to achieve the same performance
-driver support is still glitchy
-*ahem* have you got the money for that thing?
-no games ect. are out yet, which would make it worthwhile
As for partitioning:
-Set aside a partition for all your data, books, music, videos ect. - that way a system crash will be a lot easier to recover, sine you will be able to format the system partition.
-You may consider installing all your programs on a separate partition, that makes the system partition less valnurable, and makes it easier to indentify programs later on.
-Putting swap/paging/virtual-memory files on a separate partiotion is also a good thing. Though win32 is nowhere near as good at it as linux, it still helps to separate it from the rest. (While you're at it, set virtual memory to a FIXED size. This will cut down on fragmentation and even out performance).