I think people over exaggerate the crossfire issues though (same with SLI) there a re very few games these days that have trouble with it, so i don't think thats going to be an issue.
the reason i dislike the 480 is because of the heat,not of just of the GPU, but of the exposed plating which radiates into the system. i just can't imagine its healthy.
and while you are right about being able to SLI, it would take a massive PSU to accommodate it, and one hell of a good cooling set up.
of the two, i would easily prefer the 5970. cooler and more powerful.
but yeah, as long as a dual card setup is viable for the user i would say a 460 setup would be a great choice. but without a capable Mobo thats just not going to be possible, in which case i would suggest the 5970.