I have never seen this type of question before as there are so few boards that can run both AGP and PCI:e. But I very much doubt that you could get the 2 cards to run together very well, as the PCI:e runs at a much greater bus speed.
I have two dual card boards, one of each (SLI and Crossfire) and IIRC the crossfire master card sets the speed of the slave i.e. to its speed. I say crossfire because I looked into it more because I run my RDX200 over the nF4 SLI D everyday.
Even with the new cable less crossfire that my x1650XT has I still think that you need cards that are quite near to each other.
Then there if the fact that do you even need SLI ?
Everything I have read about it says that it only comes into its own when you have a very big screen resolution.