I might be wrong, but I think that you can do it (without the bridge between the cards). The cards can technically run in sli without the bridge, but performance is poor because the pci bus becomes saturated. I read that in this month's maximum pc.
However, chances are that if you have a current gen pci x card, then it will have dual video outs. Unless you want triple or quad monitors, you shouldn't need two pci x cards.
Two pci x cards of different types should, in theory, work. Remember the good old days of running an agp card and a pci card to have a 1337 dual 17" monitor setup?