Unfortunately Nvidia cards need to be hardware overclocked. It can be done, check out one of
the many PC overclocking sites. I can be done and I would go ahead with it, but you are probably
better off buying a new card.
You can find an ATI 9000 Pro 64mb for cheap and overclock it
with software. You could also get a 9600 Pro 64mb for 100 bucks from OWC and mod it to work
with your mac (I think, I dont know which one you have). You can overlcock it with software too.
Hope that helps. If you can find your card on a PC site, let me know, Ill dig it up.