32/64 bit Vista? Or XP? (to and from)

Well, you would need to format your hard drive, so you would need to reinstall literally everything. And it is so you can have more than 4GB of RAM (the max the 32bit Windows OS will recognize, actually 3 point something, but I am too lazy to look up the exact number). You would lose your save games unless you are able to copy the physical save game file from the game. You would need to look online for each game to see if it is possible to manually make a backup of a save game. With some games it is possible, with some it is not.

Be aware that in 64bit all drivers must be WHQL digitally signed to be able to install them, aka, the driver must have been submitted to Microsoft and Microsoft have signed off on it. So if you have 3rd party hardware, check the manufacturers website to see if they have a 64bit driver for it!!! Printers, scanners, cameras, everything.

Make sure you backup all of your data, and make sure you have the install disc or executable and all required license keys for all software that you want to put on the machine afterwords. Also be aware that some programs will not run in 64bit. I have had programs literally pop-up a box after the OS detection part of the install and tell me that their company does not support 64bit Windows, and I can not install. Some older programs that still have 16bit code in them will not run either.

Other than that, it is not too much different than a regular Windows install. Just be prepared for potential pains in the ass until the rest of the world catches up to 64bit OS'.