The fastest Pentium 3 you can get is a 1.4GHz. However, depending on your motherboard, you may be limited to only 1GHz, due to a newer core for all PIII's above 1GHz (1.2GHz, 1.4GHz, IIRC)
You can go to a 1.5GHz chip, but you're either going to have to get a new motherboard as all CPU's that run 1.5GHz and above have a different socket than the Socket 370 or even Slot 1 that your PIII has.
Additionally, you'll be looking at new memory (unless you want to castrate the performance of the CPU) and likely a new power supply.
Also, while 98 is fine for what you have, you're going to find that support for it in drivers is waning, so with XP available, you should really consider switching over. There is also an issue with the speed of the faster CPU's (around 1GHz and up) that can screw things up. There is a patch for it, but, unless you have some sentimental attachment to 98, you're going to find that going to XP is going to be a better move, especially since it'll take better advantage of that much RAM.