GFX Cards.

Should I sell my 970 SSC and get a 980 TI or buy a second 970 to run in SLI?

 

The way multi-card set ups work is that the load is shared while in a game. It doesn't literally double or triple your power - it only uses a little at a time from other cards if it needs it. To be able to draw that power, the software has to be compatible. The engine the game uses has to be able to take advantage of multiple cards, and even still, some tasks will be assigned to one card and not another as opposed to being shared between the two. What you're left with is not necessarily a better system overall, but rather a system that can take one resource-heavy task and spread it across multiple underperforming cards.

With a single-card setup with a better card, you're assigning all resources to a card that has better texel rates, pixel rates, memory bandwidth, all the good shit. The load isn't spread, but the jobs that the card does itself will be done a lot better than a worse card in a multi-card setup.

A good analogy would be with cars. If you could put two perfectly synchronized engines under the hood, one controlling each wheel as opposed to one controlling an axle of two, your car would be able to handle a harsher load (more rugged terrain that requires more torque, etc.), though it wouldn't necessarily go faster.

So, as a tl;dr, do multi-card if you have a heavily load of instructions where quality is not a priority. If you really want to turn up the graphics settings, get better performance at higher resolutions, etc, go with the single-card build. I'd only ever really recommend a multi-card build if you had like a couple Titans lying around or something. Never on 960s or 970s.