djw2k3:
The softmod for GeForce 4 exixts. You have to use the later versions of rivatuner to get it working. see what driver versions they are recommending, but I think the 30.80 have a good reputation.
And DanT, if you are to use the computer for gaming as well, i would look seriously in to the Ati 9500/9700 series. they are all fully directx9 compatible, and should give you a system with a long lifespan. These cards are also moddableto fireGL series, but I have no experience with them my self. Heres a link to a tread with a lot of satisfied people.
Pay close attention to what kind of card you are bying, as this type of card can be modded in all kinds of ways, and some might give artifacts, or not be able to be modded to x1 level, only Z1 (this conserns the 9500 128mb cards )