id say you could stuff 4 cards in that board, but you will need some dummy vga plugs (google them) and if you would be putting 4 in there i would seriously consider going open case
I am considering doing the 4 then. 2 separate systems with 2 5970's each, is also a possible idea now. By open case, do you mean leaving doors off, fan full blast, or literally an open case design?