My home is kept at 65 degrees in the winter and I managed to get a constant idle temp of 22-26c with all voltages turned way down and CPU speed at 1.5 Ghz. I also noticed that I see no difference surfing the net or operating my computer (except for gaming) at these low performance profiles. It says a lot for keeping a computer overclocked all the time. Just will heat up the CPU for no good reason for no improvement in 2D performance. This is with a i7 920 processor and an H50 Corsair water cooler. I am a believer about the new intel enhanced speedstep. It boosts up to 3.6 as soon as I put on a game but otherwise keeps the system at 1.5 Ghz.
I think it depends on what your general use is, but probably. There have been a few articles on Tom's that go into what speed is the most efficient under load, but really if you're never taxing your processor then these articles don't apply.
Not only $500, but the larger electric bill as well. Depending on your game and resolution, you could probably do quite well with the 5750. Around the speed of the 4850/GTS250, but much more power efficient.