Hi, there is no major issues really, from having had a mixture of multiple cards in my system past and present, my experience has taught me this:-
Nvidia + AMD:- works well, but only on Win XP and Win7 as Vista does not support multiple graphics drivers to be loaded at once, when Nvidia drivers detect an addtional alternative graphics card that is not nvidia also, it will disable CUDA and PhysX support at a driver level on the nvidia card, although there is a
PhysX driver hack that bypasses this check and thus re-enables it.
This is handy for any games that use PhysX, while you render the game on your AMD card, the physx calcualtions are offloaded onto the nvidia gpu lowering your cpu usage which can give you extra performance on games that support it.
Nvidia + Nvidia:- Try to keep the cards directx support to the same level, ie dx 9, 10 or 11.
AMD + AMD:- Try to keep the card series the same for eample, any HD6xxx, again it is best to keep the same directx level.
EDIT:- I should add that using any addtional cards/usb devices to connect a 5th monitor will not be compatible with eyefinity for a 5 screen gaming setup, for that, you would require a 6 port card, or alternatively you could use the
Zotec DP > 2xHDMI device, it acts like a Matrox DualHead2Go device where it spanns the display across 2 monitors.