you cant use an AGP and a PCI-E card together, only one will be active at any one time
The only other option is to have an SLI board which will have two PCI-E sockets to take two (identical) Nvidia gfx cards. Two cards will offer an improvement over one (its not double the power but its a good increase). However with the advent of some of the newer cards be warned that two is not always better. For instance, having 2 7600's is not necessarily better than having one 7900.
You can also have two ATI cards in one board, instead of SLI this is called Crossfire but works on the same principle. The difference being i believe that in crossfire you can have two different models of ATI card (although they will need to be crossfire compatible)