if all you care about is your data, and you gonna put the new hdd in the same box, first take out the old hdd, put in the new one and install 7 then put back the old hdd and select the new one on the bios to be the first boot device then inside 7, go to your old hdd and cut and paste favorite, desktop, my documents folder as well as any other folder you have loose somewhere on your disk.
then format the old one and you have an extra hdd on your machine.
otherwise go into win xp and start >all programs >accesories >system tools >windows easy transfer
that copies all your data as well as your settings, but i think you have to use an external hdd for that...
also:
http://www.microsoft.com/downloads/details.aspx?FamilyId=2B6F1631-973A-45C7-A4EC-4928FA173266&displaylang=en
http://technet.microsoft.com/en-us/library/dd446674(WS.10).aspx