Looking over the results, it appears the card only shows promise in 3DMark06, edging past the GTX 260, but in all the other tests it struggles to keep up by a good amount, making it more of a comparable offering performance wise to the existing Radeon HD 4870.
Source: http://www.tweaktown.com/news/13287/radeon_hd_5770_benchmarks_online/
http://en.expreview.com/2009/10/08/asus-radeon-hd-5770-benchmarked.html/2