I guess that depends on what screen you have, if it's something up to 27" (1080p or even 1440p) your are fine with just solid single gpu and definatly GTX 970 is one of these, but if you are planning to get something bigger that supports 4k resolution then you might want to consider going for sli/xfire.
For now I am not sure if drivers support is good enough for that, but I never actually had a chance to test it so you may want to ignore that part.
About cases, I own Fractal Design Define R4, and the only thing I hear is silent whisper of my case fans, which is great imrpovement from my old case which was driving me crazy when fans started to buzz. My CPU cooler is rather shitty but still can't hear it. But it has one downside... it's huge and pretty heavy but that actually says a lot about build quality. I think it cost pretty same as Corsair Carbide 400R so you might want to check if you like the look.
+1 for the Fractal, I have a Corsair 300R personally, a tad noisy, but it doesn't bother me too much.
However the Fractal is incredible, it doesn't have the best airflow, but it's a great looking case and extremely quiet. (when a friend asked me to spec him a pc like mine but better, I suggested the R4, he loves it)
I currently use a 28" 4k monitor, I found it was possible to play a surprising number of games at 4k on a single 780 (30+ fps). I have since gone SLI, I can't say I've had more issues since going SLI, but it is an extra layer of stuff that can go wrong. Frame-pacing is fine for those interested.
SLI scaling isn't as good as xfire, but it's smoother imo. I do have to lower settings on some games to run at 4k, SC in particular requires I drop the resolution to 1440p. (Until they improve SLI support, I can run most the crysis games fine at 4k)
If you plan on 4K, I'd wait until the beefier cards arrive. Otherwise, a 970 should be fine for all applications.