Yes you can, as long as its the same GPU.
e.g.
79xx will work with any other 79xx
78xx with work with any other 78xx
cannot mix 78xx with 79xx
the fastest card will always slow down to match the slower card.
e.g. crossfire 7970 with 7950 and its just like having 2 7950's.
this goes the same for 2 7970's with different clock speeds or VRAM. the bigger / faster one will always drop down to match the slower.
The Brand of the card does not matter at all.