Have a look at the following...
http://www.racesimcentral.net/
There is a utility there called the "Gamers assistant" which seems to allow
you to overclock TNT based Vid cards.....havn't tried it myself yet - I
think a fan on the NVidea chip would be a good idea first!
Use it at your own risk!
For Diamond V550 owners there is a link to Microsofts 252 drivers which seem
to give a good uplift in performance anyway.
Iain