If you have the latest drives 80.xx then you can use two cards from different manufactures - get the driver first. They can even have different clock speeds and memory sizes but MUST be the same core.
6800 & 6800 is OK
6800 & 6800GT is not OK.
Note: NVIDIA recommends that SLI customers, whenever possible, purchase graphics cards with matching memory sizes. When dissimilar memory sizes are enabled to work together using Coolbits (value set to 18), the effective memory size for each card becomes the smaller of the two memory sizes.