How do I go about changing my windows XP operating system to Windows Vista? Im upgrading my whole system.?

Upgrades include a new Nvidia 680sli motherboard,
4 GB ram, and
BFG Nvidia GeForce 8800 GTX OC 768MB graphics card (possibly x 2).

(What else will I need? I.e new case with water cooling, PSU etc - bought my PC from currys 2 years ago - fairly dated)
Update: Only reason for changing OS is that graphics cards were built for Vista - would this matter??

Also if i do change, what is the difference between 32bit and 64bit?
Update 2: Taking your advice I think I will stick with XP.

However as I am buying a new motherboard - will I still need to buy a new Windows XP disk? (for the installation of motherboard)

When I bought PC from currys I never received a Windows XP disk just some Packard Bell recovery disk.
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