You will need to find out the limitations of your old motherboard BIOS. And the OS in use.
For example, you need XP SP1 or above to support drives over 127GB. Win98 isn't going to hack big drives (can't remember the limit.. but it is comically small by today's standards... IIRC Win95 couldn't hack anything above 2GB!! (or was that 4GB?))
Also many old PIII motherboard had a 32GB limit to the drives they could support.
Suggestions:
You could buy a new PCI IDE (SATA) controller for that old PIII (cost approx £20-£30). This will then allow you to work around the BIOS limits. And add any sized hard drive.
Best bet? Buy a USB 2.0 PCI add in card. Cost around a tenner. Then buy a big hard disk and put it into an external caddy. This then means you have the old, small hard disk inside the PC for your OS and Apps. And you can have a fat external storage disk for all your music and pr0n.
This solution also makes it easier for future upgrades, etc.

Also gives you a few spare hi-speed USB ports for your flash drives and MP3 players.
It could be a right headache trying to get a big drive into that old PC.