Just thought I'd start this thread as 99% of the posts on this board seem to be from people asking why they are getting poor hash rates on a Nvidia GPU.
Dear people with Nvidia GPUs. Your hardware sucks at bitcoin mining, period. Yes, it might be a very fast and powerful gaming card, and yes, when it comes to folding, Nvidia GPUs always beat ATI. But for this particular task, ATI cards are far far far superior in every way.
If you really seriously want to try to make some money mining, sell your Nvidia card and get an ATI one.
Put it into context. A Nvidia GTX580, top of the line card that costs $$$, is roughly about as fast as an ATI HD5750, a budget card you can pick up for less than $100.
You could sell a GTX580 and buy 2x HD5850s with the money, put them in crossfire, quadruple your hash rate, lower your energy usage and still have some cash left over.