Concerning two 460s vs. a single 570. If they are comparable? Yes and no. In benchmarks they may be somewhat similar, BUT you will have two graphics in your system, which brings up several problems. One of them is airflow and temps as you said already. The other one is SLI, it works great in Benchmarks, but in real situation a single card will save you a lot of trouble. SLI and Crossfire are known to be a real pain in some situations. And since you were talking about future-proofing your system I would ditch that SLI 460 idea and throw in a single 570, because it offers so much more and the same/lower price.
Are you essentially saying that in an ideal situation, SLI would give you that much more of a performance boost, but in the real world, it just complicates things and provides for another thing to go wrong. I would see a difference in benchmarking, but in real life applications, the difference, if any, would be outweighed by the disadvantages? I believe in my pricing, the SLI configuration would substantially be more expensive than the single GTX 570. Thanks!