Just make sure you backup your data. There should be no problems, you should be able to run the Windows Upgrade Anytime site. Are you using the orginal Vista media or OEM.
Also are you upgrading for a reason. Have you got home or premium right now, I think you said in another post that you have home.
If not, there are very few features in Ultimate over premium worth paying extra for. If this is to do with joining the domain then I seriously recommend using VMWare and just building seom WinXP pro virtual machines.