Hello, what graphics cards are you looking to Crossfire? There is not a major performance difference with x4 and x8 especially with the smaller graphics cards.
I have two 5770's the x16/x4 combo doesn't seem to be good at managing 2 cards, especially when I see the two cards doing this:
GPU1 load:70% GPU2 load:99%
when running games
crossfire people say that crossfire doesn't like mismatched bandwith, that would probably explain why I see flickering when I run furmark >_<
(well not really flickering but the textures/text seem to be somewhat flickering, kinda resembles when the TV has a weak signal)