750 sounds good but without knowing the amperage distribution not very helpful, are these still equiped with the raid drives? One card horizontal is a given, two might be possible depending on the specifics of your units.
Agreed with Deslok about the power.
If it looks like the chenobro at:
http://usa.chenbro.com/corporatesite/products_detail.php?sku=19Then I'd say (just from eyeballing it) you could likely get two video cards "floating" horizontally over the motherboard if you could rig up a bracket. I don't know about more, would depend on the card, and if it is in a rack or not (if you can cut a hole and vent heat out the side or not :p)