I've installed XP Home and Pro on dozens of computers and never had to call. Most of the keys were from junk PCs that were on the curb for collection, so they were even being installed on different PCs.
Online activation was all I needed to do.
As far as your situation goes, I think it's a bad idea to set up a second partition and try to dual boot two copies of XP. There's just no point. If you are concerned about performance, back up what you need, wipe the drive, and do a fresh install with a SP3 install disc.
