but if you have a crossfire board, you cannot SLI on it... that would require the x58 chipset i believe? to go wither crossfire or sli at your chosing that is
x58 chipset is the chipset on Socket 1366 Boards (Intel i7 9xx processors, triple channel memory). 750a, 780a, 980a are Nvidia chipsets for AMd supporting SLI on AMd motherboards.
If you must crossfire rather than using a single card, I would take the 6870. It is the same or better performance in near enough all situations as a 5850, however the 6xxx cards scale much better in crossfire than the 5xxx cards do, so you will get a much larger performance increase with multiple graphics card setups with the newer 6000 cards than you will with the 5000 cards.
Remember though that the numbering system is different. Although the 6870 outperforms the 5850, a 6870 is a mid-high end card, like the 57xx cards were, and the 69xx will be the high end cards, like the 58xx were, not multi GPU cards, like the 59xx cards. If you have a higher budget, wait for the 69xx cards to come out in a few weeks and see how they perform. Chances are a single 69xx card will massively outperform a single 6850 or 6870, and be only just beat, if they are beat, by 2 6850's or 6870's, so 2 together would be insane performance.
For which cards work together, they have to be the same chip. So, for example, you couldn't crossifre a 5770 and a 5850, because they are running different chips. You can, however, crossfire different brands, so you could have an XFX 5850 with a Sapphire 5850 for instance.
The same is true for nVidia, you can't SLI a GTX460 and a GTX480, but you can have an EVGA GTX460 and a Palit GTX460 in SLI
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