It will ask you for your Win98 disc... though to be honest I'm not sure if that qualifies as an "upgrade" in their mind. In some cases they have only allowed you to hop up from the previous version.
Back everything up and see what happens.
Actually now that I think of it, I upgraded my admins desktop from 98 to XP... shouldn't be a problem, just make sure you have a 98/2K disc handy.
P4 2.6C, Thermalright SP94/Panaflo 92MM M1A, IC7 Max3, Swifty MCX159, 1GB Geil PC4000 Ultra Plat, Radeon 8500, Audigy 2, 2X WD Raptor RAID 0, WD 250GB SE, Plex 708A, 166SDVD, LianLi PC75, XP Pro.