Dunno if two 970s are a good idea from a cost standpoint, but I'm going to assume there might be other reasons like a certain efficiency requirement, or preference for Nvidia drivers, or that you just have done your own research, and I haven't looked into SLI because I'm one-card person. I'm not playing at 4K so I don't need two 980s or two 290Xs.
That said, one 970 will pretty much play every old game gog has at 4K with some heavy AA. Pretty much anything that released up to the last console generation should play 40+ fps at 4K with MSAA. Gog doesn't actually hack the games' binaries much, and certainly not to the point of adding multi-gpu support. Aside from that, most games' multi-gpu support comes from code branching in the driver when the driver determines what game is running. Nvidia will likely not have written SLI profiles for old games. You can ask them if you want to be sure.
As I don't use dual-cards, I have no idea if you'll have to toggle one card off before you play an old game to avoid a crash, if it'll just go with both cards in a default profile that will cause some stuttering, or if one card will just get used and the other will idle. Best email Nvidia support about it and check on forums.
But rest assured, from the great majority of releases, and even for the ones that gog has more of a hand in themselves, there is no SLI or CrossFireX support in these old games. Not that you'd ever need it, even at 4K, with a GPU like you're thinking of buying.
Ohh ok I see now that you have a 144hz monitor. My 40+ fps estimate was a bit conservative because I don't know how newer, middle-indie, less-optimized games like Starpoint Gemini 2 or Shadows: Heretic Kingdoms will run, and wasn't comfortable saying 60 fps. I was also assuming 4K. But at QHD, I'm pretty sure one 970 will run every gog game at 60+ fps with AA, excepting The Witcher 3.
Post edited April 20, 2015 by johnnygoging