Stick with windows xp for a while.
When i tried vista a few weeks ago my newish x-fi sound cards software(the control panel) wouldnt install so i couldnt tweak my sound settings for best sound, and also my labtec webcam wouldnt work even though i downloaded the vista drivers, so i got abit cheesed off and reinstalled win xp, its nice to have everything working agian!
P.s no you wont need any other software for the upgrade, just make sure you have an xp cd to reinstall windows and then your good to go. Only things that would require extra software is hardware like graphics cards/sound cards etc, which you'd get in the box and you just install, so dont worry about having to buy anything else other than win xp if you dont already have it