Originally Posted by saulin
Because as soon as you go too high on the 6990 it seems to downclock. Also the 6990 is not a good overclocker by any means. Any reviewer that got it as high as 1ghz had to increase the fan speed to 100%
I wasn't talking about air cooling, but to sum up the air cooled facts...
The 6990 will go to about 950MHz on air before you have to go crazy with the fan... but the VRMs seem solid (I think it is only 8+2 but maybe they are just better quality?). Most sites are reporting 675MHz-750MHz on air for the 590. Any higher and you risk hitting the OCP limit. Fermi draws A LOT OF POWER. At those clocks you will have a little more power with the 6990 especailly at huge resolutions, but it will be loud. The 590 is slightly slower but quieter... This the air cooled conclusion -- and I think we all understand this point by now.
If you put both cards under water 1) 6990 fan noise is not problem
2) you may run into issues if you overvolt too much on the 590 -- you will either hit the OCP limit (450W) or blow the card if the limiter is disabled. Nvidia has acknowledged this themselves.
Basically, somebody needs to put out a good non-ref power design to really unlock the 590 clocking potential (under water) whereas the 6990 power design seems
fine even for extreme clocking (again I am not talking about air cooled here)