I am currently running 3 monitors at 5040 X 1050. My current system is 8GB of ram, i5 3570K (stock) and a radeon 7870 GHz. At this resolution, I can pretty much max everything out, but in titles such as Battlefield my frame rates seem to take a hit when there is a lot going on. Would running another card is crossfire help this at all or am I running into a VRAM problem?
you are running low on vram since you r running multi monitor.
setting up a crossfire won't give you noticeably better results since you can only increase the processing power of the gpu but the vram still remains the same because both the cases have to essentially load the same amount of textures.
crossfire ing 2 cards does not mean 2xVRAM of 1 card.