It might require to turn off 3D acceleration on one card, or it might very complicated to sort out DLLs by hand if you want to start 3D apps on one display or the other.
I ran tri-head with a 8500LE AGP and a FX 5200 PCI but that was under Linux where things are a little more straightforward.
I had to kick out the ATI crap for other reasons, though.