If the heat/power/noise issue doesn't concern you then the GTX 470 is a good choice. You do mention SLI though and you must keep in mind that SLI will make the heat/power issue twice as important.
If you are using 1680x1050 then really I would just get a GTX 460. It should be extremely good at that resolution and really quite good for 1920x1080 as well, especially if you OC the card. If you feel the need for something better later you can get another for SLI without turning your computer into a space heater. At stock two GTX 460s come quite close to an HD5970 due to SLI scaling better than crossfire.