To solve the problems of the upgrade paths,you can fit the hard drive as a slave to another PC or use external convert cables and format as a slave.Refit as main drive then you could install XP Home it is a little cheaper than XP Pro.

$50 more for Pro over home oem on newegg (do not know of anywhere else to look seeing as im the other side of the pond)

Im guessing an upgrade cd will cost more or less the same as an oem as they carry a retail price iirc (do not think oem upgrade discs exist but may be wrong)
I personally would back up all data you require and start again from fresh anyway as upgrading an OS is frought with possible problems ie chipset driver conflicts etc however often generic drivers in xp should replace these if conpatable and at least 2000 is NT based I would never upgrade a 9x kernal to NT kernal

Marked as Answered.