I voted 10-15%.
A few months ago I said the GTX480 would be around ~30% faster than the 5870. What changed now is that ATI has been slowly improving performance with each monthly driver, and since the release of the 5000 series 6 months ago, I believe they added on average 5-10% more performance. Additionally, we have the rumors about the GTX480 being a 480 core part, instead of having 512 cores. So it loses some power there (-6.6%)
Anyways it should be a powerful card, and may be noticeably faster in some cases, especially situations that require more memory and/or higher bandwidth.