I am using 2 cards in my system:
AMD Radeon HD 6450
NVIDIA GeForce GT 520
I was using two far more expensive Radeon cards, but the fan on one became very noisy and I "leant" the other one to my brother. Anyway, what I have works for what I do, which is Visual Studio mostly.
The only problem I found is the cards do not support more than one digital output on each card. I have found that VGA isn't so bad for 2 monitors when I have 2 with digital rendering.
My mother board is an ASUS P6T. I could even add one more graphics card, but I seem to have the sweet spot of 4 monitors and haven't yet found the need for even one more, but only 3 would suck.
Sorry to take so long to answer, but I didn't notice until just now.