5970 and 480 are not even in the same class performance wise or pricewise. THe 480 more compares to a 5870 in both aspects.
That said, the gtx480 is slightly faster but a good bit (generally 100us) more then a 5870 but with that extra speed you have an insane TDP, power requirements and heat generation. MANY schools of thought agree that the 5870 is a much better option for these reasons, even though it is not as fast.