Firstly have you got copies of your drivers for your hard where?
Secondly is the xp pro a full version or the up-grade version?
If xp home has been pre-instaled, a back-up copy might be located on a hidden partition of your hard drive. Check your manual to see if it's here.
In theory you should be able to carry out the up-grading version of xp pro over xp home, and it should only alter what it needs leaving your applications intack BUT, I personaly wouldn't trust this method 100%.
A complete new installation of xp pro will delete all non windows applications I.e office ect.
Before starting I would ensure that you have a copy/back-up of all your hard where, graphic card, sound card, modem drivers etc. make back-up copies of all your important data.
Having only a pre-installed xp, would mean that if things do go wrong, you might not be able to restore this with out a restore disk, also the pro version doesn't like some unsigned drivers.
Another thing to concider as there isn't alot of difference between the two operating systems, do you really need the up-grade, if you are using it at home on a single machine or small home net work, the the extra bits you get aren't really worth the extra exspence.
Oh, a up-grade version will always need the copy of the lower version every time a reinstall is called for.
I hope that I haven't confused you too much, and this is of some help!