The 5770 performs about the same as a 4870 in both Toms and Anadtechs reviews. That probably means right about the same once the drivers get optimized. It costs a little more for the extra features. If you are going to make a performance failure claim, back it up with something.
Hope I did a good job writing this.......http://www.bit-tech.net/hardware/graphics/2009/10/13/amd-ati-radeon-hd-5770-review/1
"In our performance evaluations, the Radeon HD 5770 performed well, but didn't set the world on fire - the comparative lack of memory bandwidth severely limits the card's potential when compared to the Radeon HD 4890, which has the same engine clock as this new mid-ranged 5000 series card. What's more, we found that the card was also slower than the GeForce GTX 260-216 and Radeon HD 4870 1GB - especially when anti-aliasing was enabled - but it does manage to keep its head well above the Radeon HD 4850, though."