Two HD 5770 cards perform better than a single HD5850 card and two HD 5850 cards perform much better than a single HD 5770 card. Considering the price of the two HD 5770s is nearly the same as a HD 5870 card I would suggest buying a single HD 5870 or HD 5850 depending on which price range. On the other hand, if you only have $160 a HD 5770 added to the system later will improve performance immensely in most applications. Two HD 5750s in Crossfire mode make for an attractive upgrade as well if you only have $130 to spend on a video card today.
CrossfireX is also available for those wanting to Crossfire three or even four video cards for even higher performance in games. CrossfireX also provides extra image quality with 16x AA available to two cards. The bottom line is when CrossFire works it works well, but a lot of its performance is out of the hands of ATI and in the hands of the game developers who can choose to support this feature and make it work properly or plainly do not support killing its true potential. Hopefully ATI will make driver patches to help performance in games that do not support CrossFire out of the box. At this moment any two ATI cards you buy no matter who is rebranding them will work together in Crossfire mode.