If it consumes as much power than it generates the same heat.
The chip itself might run as a lower temperature, but that is due to the cooler differences. The only difference is where the heat ends up - in the GPU, inside the case, the room etc.
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(the following is not a reply directed to the quoted person)
If I remember correctly people play CS at 800x600 to get a larger crosshair or a larger aiming point or something along those lines.
Regarding the OP question if the AMD cards can or not run displays at non-native resolutions, I think it is possible, but not entirely sure.
The 6870 consumes less power than either GTX470 or an OC GTX460 so it will generate less heat, but the GPU itself might run hotter.
So if you are concerned about the GPU running at a low temperature, then the GTX460 is generally a better choice, although some custom cooling GTX470 or 6870 might run as cool or cooler. If the problem is the environment around you warming up then the 6870 is the better choice.