Same dual graphics cards, or mix and match?

Is it better, or even possible, to mix different graphics cards with a mobo as long as both are crossfire ready? Or is it more wise to stick with two of the same graphics cards if your going to run duel graphics cards.
 
I know some people who say they run duel cards, It seems to be a very good thing. But I have heard of them only running 2 of the same, Not 2 different ones.
 
SLI = 2, 3 or in specialised cases, 4 nvidia GPU's run together. Must be the same GPU (processor) - so basically the same model cards. The can have differing amounts of memory but SLI will default to the lowest amount on either card. Available on most older nvidia chipset motherboards, some AMD motherboards, and most multi-GPU capable Intel motherboards using the P55 and X58 chipsets

Crossfire= 2, 3, or 4 AMD (ATI Radeon) GPU's using the same processor. Unlike SLI, the AMD cards can be mixed to a degree. Cards of the same model line can be used in conjuction, so a HD 5850 can be run with a HD 5870 for example, or an HD 4850 can Crossfire with a HD 4870. Available on most modern AMD and Intel chipset motherboards

Lucid Hydra = 2 (at present) GPU's that can be of either/both AMD and nvidia. Can only be used with Windows Vista/7, no dual-GPU cards can be used and supports cards no earlier than AMD's HD4000 and nvidia's GeForce 9 series. Very few motherboards (all MSI at present) support this multi-GPU technology

GPU = Graphics Processing Unit. The graphics/video card equivalent of the CPU. Most cards have one GPU, but some cards utilise two GPU's.
 
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