On the other hand, there's basically nothing that exceeds the capabilities of your cards, at least at single-monitor resolutions.
That's not exactly correct. The GTX680 is about equal to my 5870 crossfireX set up. And it's not all about a gain in performance. I don't keep computer hardware to the point that it has no monetary value. I could get about $350 for my 5870 cards if I was to sell them now, come summer time, that will be down to somewhere between $250-300.
BTW, a 30% performance boost is plenty for an upgrade IMHO. Especailly considering I'd be getting that boost from a single card I'll SLI as a later upgrade. Also, keep in mind this is the monitor I'm using- http://www.newegg.co...N82E16824001098