I have a GTX 780, i5 quad,win 7, 16 GB machine. I had to change the primary video device in the BIOS to be the GPU in order for UE4 to work. So my VGA is plugged into the GPU. The problem is when I do viewport renders (even very basics scenes) the computer is extremely laggy, and effectivly unusable. Is there anything I can do beside going back to the onboard VGA (Which means UE4 won’t work)
The best solution for your situation would be to get another GPU (if your system can handle another one). Otherwise you are just going to have to deal with the slowdown when your GPU is being shared by both the desktop and Cycles. 
It doesn’t have to be an expensive GPU, just one that will support what you want to do with UE4.
Ok, thanks. I will look into a second GPU, I have one more PCIe slot. I guess I’m trying to figure out which one makes the most sense, and I’m not sure how cycles will use them together.
You can use them together if you wish, but then you will be in the same predicament that you find yourself in now. For example I have an old GTX460 that I use to drive my monitors and desktop, but my GTX980 is not connected to anything so I can use it’s full potential when rendering. While I’m rendering on the 980, the 460 is completely dedicated to my desktop, and whatever I’m running at the time, so no lag. I can combine the two to get an extra render boost, but it will slow down my system. I only do that if I’m rendering an animation, etc. that needs more rendering power than just one card can give me. Although the 460 doesn’t really add much to the mix as it’s so old, slow, and only has 2Gbs of vram. 
Ok cool, thanks.