correct me if i am wrong but a GeForce 4 is an overkill for a PIII 600 Mhz processor. I think that you need atleast 1.0 Ghz to get the full performance of GF4s.
i think that your best bet would be to get a 64 MB GeForce 3 TI 200 or ATI Radeon 8500 . They should cost you around ~90 shipped from most places nowadays. I would pick one of these two cards, perferably the Radeon. Any game released upto right now and in the near future will run with excellent quality on these two cards.
If you want to spend less money, then Consider GeForce 2 TI or ATI Radeon 7500. they should cost ~55 and most games will run fine on these cards but latest games might be little choppy.
Dont waste your money on a GeForce 4 because you wont be able to get the full performance of it b/c of 600 Mhz processor as i said earlier. But, if you are planning to upgrade your system in the near future, then you might want to buy the GF4 and use this card in your new system later.
hope that help
thanks
gsethi