Why upgrade? If the drivers are running stable, upgrading to a 9x driver might get you a frame a second faster, but probably some crash issues.
To be honest, NVidia tends to continually update drivers for the newest cards, but older cards rarely get huge boosts with newer drivers... at least that's been my experience.
Having said that, I'm still using 82.12 drivers which I found to be very stable with my 6800GT in WinXP.