cheapest way would be to remove the HD out of your 98 box and slave it into the new XP box and just do HD to HD transfer, then wipe that HD out, and keep it in there for back ups, or take it out and trash it.
The fastest thing would be to slave the drive in the new pc i did a transfer over the network using workgroups and if you are going to be transfering a lot of files then do not try to do the network unless you want to wait a day lol