Only on the one machine, not the two. Ih you want it on the two you'll have to buy two copies.
But if your happy with what you have, ie: it works perfectly fine. Then dont upgrade. XP is far more stable than 98, so if you want stability XP is then one.
Also if yo go XP you'll hae to check that hardware and software is compatiable, worst case you'll have to buy new soft/hardware to run on XP.