When you buy multiple video cards to Crossfire or SLI do they have to be the exact same model and/or brand name in general or can you use an Asus with a Sapphire, for example, with no problems? If not, is there any advantage to using the same exact card you bought before or the same brand name?
Also specifically how much power in watts in necessary for both the 6850 and the 6870 when you have them in Crossfire?
They generally should be of the same model. You can mix brand names.
Generally, it is best if both of the are the same like two HD 6850s, but you can pair up a HD 6850 with a HD 6870 if you wanted to. However, you'll be wasting some of the HD 6870's performance. Also, if one card has 1GB of VRAM and the other has 2GB of VRAM, then that extra 1GB of VRAM on the other card will be wasted since games have to load the same textures into each video card.