120Hz just isn't going to happen any time soon, at least not if you're talking about resolutions in excess of 1080p. Some games would work fine but most multi-GPU setups wouldn't be able to pump out the framerates required at 1440p, let alone 4K. You either go for a high resolution and sacrifice the refresh rate or go for the refresh rate and sacrifice the resolution.
Even at 1440p a dual 290X setup can only manage 90fps in BF3, 70fps in Metro: Last Light and 70fps in Crysis 3. Given the overhead for processing the two-images (they are calculated from different perspectives) a 1440p display will likely struggle to hit 60fps in a lot of games.
Sadly I think 1080p@60Hz is all we can expect given current GPU limitations, otherwise the potential audience is just too small (and I say that as someone who games @1600p).