The biggest tip I can give you is to make sure you have backed up everything from your current XP installation. And I mean everything.
Especially check c:/documents and settings/user name/local settings/application data/... if you have Outlook (you will have to enable viewing of hidden folders to see this folder). Certainly backup everything in user name/...
Do a thorough search of every folder for anything of importance. And do this twice. Also make sure you can restore XP if Vista does not work for you.
Personally I would install Vista WITHOUT your serial number the first time. You may have to upgrade some hardware to get vista to work properly, and this may included your motherboard. You only get one shot with the motherboard and Vista OEM, so best to be on the safe side.
Finally, have I stressed how important it is to make sure you have backed everything up. Good. You wouldn't believe just how important this is... Often until its too late.
Although it's not really for me to say as its OT, but I hope you enjoy Vista better than I did.