Well, the 5800 would run better than my 5600, although not a huge step, but still a step in the right direction.
I owned a MSI 5600 before I got my PNY 5800 Ultra and it was a very huge step..
As for why I want another one? Its still a nice card and isn't leaps and bounds faster than my 9800Pro aside of newer DX9 games, and I still find nvidia drivers way more stable and bug free. PLus, its a collectors item