I have a 5 monitor setup at work and would like to go to Ubuntu, but only if I can get all of my displays to work. How many monitors can Ubuntu support at one time??
Theoretically as many as you have, but practically you could face problems...
I had problems with a PC with one ATI and one NVidia graphics card for 4 screens.
With two NVidia graphics cards I got it to work with Twinview using the proprietary driver. With ATI only configuration this should work as well, however this is not named TwinView. But I did never have two ATI graphics cards - thus untested.
About other graphics card vendors I have no information.
You should add some information especially on your Graphics hardware and Mainboard Chipset.
You mentioned 5 Screens - 2 cards with 2 screens each and one connected to an onboard graphics card? This might as well cause problems depending on which onboard solution it is.
I agree with Snyperx, most onboard video adapters are automatically disabled when you add a video card. I suggest that you use a USB to VGA adapter instead of the onboard adapter.