I have a couple of 980's and they haul ass without a lot of whining or complaining, so I love 'em. The heat is not too bad, but a third card would make the home office about as comfortable as the inside of a pizza oven.
My rendering and folding seem to be compatible. Most unbiased renderers will make use of Nvidia cards, and they seem to be good folding performers, too so I'll stick with Nvidia. Folding sort of "ducks" out of the way when I send render work to the GPUs, and I don't seem to have to do anything special. I like that.
I am still thinking of building a 2P rendering box with 3 GPUs. I have a couple of choices:
1. Put the new box into a spare room, let it run F@H 24/7, then build a local cloud to get rendering work over to it and back. Some rendering hosts will allow a client to run on a remote machine, but others won't. For the less-sophisticated ones, the local cloud would let me run everything "locally" on either box and get me around that problem. Assuming that licensing isn't impacted. Something to figure out when I get closer.
2. Build the new box with 2 GPUs (still need slots and backplane space for other cards, DSP and such), and replace the current workstation in my home office. Then take that old workstation, add a GPU, refloor it in another room, and let both run F@H 24/7.
Either way, I think I can start pushing about 1.2 to 1.5 million points per day and gain a rendering rig and a home theater server at the same time.