I don't have too much experience with other GPUs, as the Gigabytes have worked well for us (if it aint broke don't fix it), but I think I've seen vapour-Xs in other people's builds.

Don't crossfire the cards. This can lead to problems and lower hashrates in Windoze and is not needed for mining. Crossfire doesn't seem to have an adverse effect on my linux box for some reason, but it isn't needed so there's no need to take the risk. If you want to use your rig for gaming as well as mining then just hook up the crossfire cables when you switch to gaming mode.