In order to provide optimal symmetric scaling, SLI technology requires the NVIDIA GPUs to match. For example, a GeForce 7800 GTX must be paired with another GeForce 7800 GTX. Likewise, a GeForce 6600 GT must be paired with another GeForce 6600 GT. However, using the latest Release 80 or later graphics drivers, NVIDIA GPUs sold by different manufacturers can be used together in an SLI configuration. For example, a GeForce 7800 GT from vendor ABC can be matched with a GeForce 7800 GT from vendor XYZ.


http://www.slizone.com/page/slizone_faq.html#c3


Clock speed does not matter:

Yes. A GeForce 7800 GTX that is overclocked (for example BFG GeForce 7800 GTX OC) can be mixed with a standard clocked GeForce 7800 GTX.

As far as I know, it will treat the faster card as if it is running at the speed of the lower clocked card, much like RAM.


A word to the wise, if you have not purchased these cards, dont an 8800GT 512, will out preform SLi 7950s, it is not worth it, you would get much better proformance from a 8800 series card, plus with the added bonus of Direct X 10.