From Nvidia's web site the SLIzone
Can I mix and match graphics cards?
In order to provide optimal symmetric scaling, SLI Technology requires the NVIDIA GPU 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 Graphics Drivers, NVIDIA GPUs sold by different manufacturers can be used together in SLI Mode. For example, a GeForce 7800 GT from vendor ABC can be matched with a GeForce 7800 GT from vendor XYZ. Can I mix and match graphics cards is one of them is Overclocked by the Manufacturer?
Yes. A GeForce 7800 GTX that is overclocked (for example BFG GeForce 7800 GTX OC) can be mixed with a standard clock GeForce 7800 GTX.
Can I mix and match graphics cards with different sizes of memory?
While it is not recommended, NVIDIA does offer this flexibility using Coolbits. When purchasing a second graphics card, you should try to match the memory size so that you are ensured full value and performance from your purchase. For example, if your first card is a GeForce 6600 GT with 128MB of memory, you should purchase a second GeForce 6600 GT with 128MB of memory. However, using Coolbits (value set to 18), customers can force both of the cards to use the lower of the two memory sizes and operate together in SLI Mode. When dissimilar memory sizes are enabled to work together using Coolbits, the effective memory size for each card becomes the smaller of the two memory sizes. Instructions to enable this feature can be found here