Thats funny XFX told me I cant run crossfire with different series cards
That's correct: you can only Crossfire within the same series like 79__, 78__, 69__, etc. so you can Crossfire a 7970 with a 7950 or 6990 with a 6970 but not a 7970 with a 6970. However it doesn't really matter since the DCS series don't implement Crossfire(I think it's the same with SLI too) and the FPS suffers if it's enabled.
Nice fps impact with CCC 12.11 beta (Radeon)
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That's correct: you can only Crossfire within the same series like 79__, 78__, 69__, etc. so you can Crossfire a 7970 with a 7950 or 6990 with a 6970 but not a 7970 with a 6970. However it doesn't really matter since the DCS series don't implement Crossfire(I think it's the same with SLI too) and the FPS suffers if it's enabled.