If memory serves, the USB support on 98SE wasn't as good as it was supposed to be - so an external drive may not necessarily work, especially as most manufacturers have stopped providing drivers for 98. That said, if you get one from (say) PC World or Argos you should be able to return it if it doesn't.
Pretty much any external disc you buy will be USB, so if your old machine doesn't have such a port you'll have to use something else (see below). If it does have USB, any sufficiently large drive will do, though I'd buy the biggest you can afford on the grounds that it would be useful for other things (like backing up your lappy). If you're really tight for cash you could also try saving stuff to a USB key or even CD (if you can).
Idea for non-USB enabled 98 machine to follow...