ASUS ROG PG278Q on the way, need video card advice

It really depends on what games you want to play. For example, older games such as Tomb Raider, Bioshock etc runs well on a single 970 (I hit average of about 90 fps in Bioshock on 1 card).

There are games that does need SLI to run smoothly (such as AC:U with the recent patch and drivers, 980 SLI gets a nice 100 fps according to

http://www.hardocp.com/article/2014/12/22/assassins_creed_unity_performance_video_card_review

I installed the game last night (finally got around to getting the codes from nVidia) and the first sequence was indeed quite smooth with SLI 970's.

Do you need to do anything to your rig to make use of 970 SLI? That'd be my main concern, if you don't need to, then I'd seriously consider SLI 970's to make full use of your Swift. However if you don't feel like spending big bucks, a single 970 should suffice. 980's price is a little too high IMO for its perf over 970. What I'd do is get a 970 now and get an early upgrade, or get a pair of 970's and stick with it for a few years.

I was in your exact boat two months ago lol.

Here was what I had considered (LONG POST warning).

My old rig was 3 years old, still runs well, and the components wasn't going to bottleneck a single 970 or 980 anytime soon (2600 on P8P67), so my upgrade path ended up being dependent on either a 24" 1080p or 27" 1440p G-sync monitor I was going to get. After some research, I came to a rather inconvenient conclusion: Taiwan only has Asus RoG Swift, none of the other G-Sync monitors are available here. An ironic situation, since BenQ, AOC AND Acer are also Taiwanese companies.

Since I went with 1440p 144hz monitor rather than an 1080p 144hz monitor, I ended opting to do 970 SLI. My reasons were, it was an excuse to find out more about and play with SLI (an expensive excuse at that), and while GPU upgrades are quite easy to perform, I am not entirely sure if I can upgrade it later (not financially, but... you know... other half...), so I went all out and got 2x 970 SLI. AMD wasn't an option, we didn't get a price drop until literally a week ago. When I bought my 970's, 290x's were still at 980's prices.

Was it worth it? For me, I think it was. I haven't played enough games to know for sure if I can put up with SLI issues, but so far I only notice microstutter when I actively search for it. For example, Bioshock Infinite's benchmark definitely shows microstutter, but in the actual game I didn't notice it, I was more concerned with not getting my rear shot out from underneath than I was with microstuttering, and I thought I was sensitive to stuttering.

YMMV, you may play vastly different games to mine, and have different expectations and sensitivities. I wouldn't say the experience is smooth straight out of the box, you may have to do a little tweaking here and there (EG monitor software calibration and LUT enforcer, running D3 in windowed full screen etc), but if you have no problems with doing that, I'd say you wouldn't be dissappointed. I put up with it as I was as eager (possibly more eager) to learn new tricks and tech as I was playing games.

I usually advocate either SLI all the way, or not at all, mostly because I don't see going SLI as an 'upgrade' path really (when one need another GPU, more likely than not a new GPU would have come out, and new GPUs usually have specs that reflect better upon the games requirements of the day). I am also biased against 980 since its price to performance ratio is quite bad compared to 970 on a card for card comparison.