Yes the 5770 will give a noticeable improvement. A vs P In my opinion its questionable if you will be able to max out, it depends on your resolution and most likely without any AA or AF. But we will see when it comes out.
Ya mien just crapped out as well, I Went with the 5850 tho. I think you should be fine, the demo doesnt even have AA just AO, VSYNC, Shadows, and Texture Quality settings.