I'm not fimilar with all of Dell's model numbers.
But since you said Win98 I'm guessing that it is PIII of some kind.
Well 128mb for Win98 is ok. But if your PC has rambus memory it will cost big $$$$ to add more.
XP will need more memory, with 512mb as the sweet spot.
Try an 80gb HD. With out knowing all of you PCs specs it may or may handle a larger drive unless you partion it.
If it is an older system it you might want to consider geting a new PC. John