It depends.
a) what do you want a new HD for? Is your current one full and you need a new one, or do you just want a faster one?
b) How quickly do you use up space?
If you're looking for a major performance boost, going from 5400rpm/UDMA33 to 7200/UDMA100 won't give you much of a boost, but if you want to spend a few extra bucks (probably $10-20 extra on a 20GB Hard drive) it can't hurt. Also, before you get the drive, you should make sure your motherboard can support UDMA 100 or 66, otherwise you'll need to get a controller card to fully take advantage of the extra speed (you can use the drive without the card, but you won't get the speed boost of UDMA 66 or 100).
If you just need another hard drive for space, 5400 RPM is probably sufficient and more practical. If you see yourself needing 45GB of space in the near future, 5400 RPM would probably be significantly cheaper then 7200 RPM.
If you have any more questions, I'll try to help.