X-bit Labs had an extreme overclocking session with the GeForce 9800 GTX. They bumped up the voltage of the chip from 1.22V to 1.528V to with the help of a 1,000Ohm resistor and bumped up the core frequency to 875MHz and shaders to 2188MHz. That's a 200MHz gain on the GPU and 500MHz on the shaders.
Surprisingly, no extra cooling was used. X-bit Labs just set the card's fan to maximum speed and that resulted in a load temperature of 85-87°C. The power consumption tests are pretty interesting, the extremely overclocked card used 182.6W, a bump of almost 70%
Our experiments didn’t go without consequences for our GeForce 9800 GTX. The card began to hang the system up after a while. The graphics core must have been irrevocably damaged somehow during the tests at the increased voltage. That’s why extreme overclocking is a dangerous sport – for the equipment if not for the sportsman himself (well, liquid nitrogen may be harmful to the overclocker, too).
So, although the volt-modding and overclocking of the GeForce 9800 GTX ensures a considerable performance growth, it brings about but small practical benefits even if you disregard the greatly increased power consumption and heat dissipation. If you could run the overclocked card continuously, the Radeon HD 4870 would still be a better buy. The Radeon HD 4850 is also good across a majority of tests. The GeForce 9800 GTX+ has lower GPU clock rates than the ones we achieved with our 9800 GTX and would have lower performance. Considering that it is more expensive than the Radeon HD 4850, we don’t think it a good buy.
Check it out over here, they have lots of performance tests too.