460 is ok if your buying 1 card at a time if you already have a SLi board, But 5970 is better from the get-go.
SLIed GTX 460s are actually equal to an HD5970 and they use very similar amounts of power(the GTX 460s slightly less.)
The difference is that the HD5970 costs $150 more and tries to cool two GPUs with one cooler which isn't optimal. If you have an SLI motherboard there is no reason to buy an HD5970 unless you don't like money. Even If you have just a Crossfire motherboard I would lean towards 2 well cooled HD5850s myself.
GTX 460 SLI performs on par with 5970 in most games, but as higher the resolution goes, the gap becomes bigger http://www.xbitlabs.com/articles/video/display/gf-gtx46...
"At the resolution of 1920x1080 the GeForce GTX 460 SLI is not that confident. Its average advantage over the Radeon HD 5970 has shrunk to a mere 3% and it loses to the latter 10 out of the 19 tests (the biggest gap can be seen in Metro 2033, Just Cause 2, Resident Evil 5 and Final Fantasy XIV Official Benchmark)"
As for 5970 vs GTX 480,
5970 is faster,has less power consumption and runs cooler too.
have you considered 3D gaming? i'm an nvidia fan, but really hope Ati bring out 3D in their 6xxx series cards!! if i were you, i would wait for the 6xxx series, just around the corner, or wait for the GTX 495 (2 x 460 - less heat and more bang for the buck!!... plus 2 x 460's perform better than the 480 for the same price!!)