Pixelmator is a nice application that claims to use the GPU for photo editing from the ground-up, since it makes use of the native Mac APIs (which in turn, are all OpenGL accelerated). The result is that most changes are instanteneous, making for a very pleasant workflow.
There is quite an increase to be make, i already knew this from video editing but it's good to see some numbers with photo editing.
Just a standard videocard is enough to speed things up at least twice as much which is quite a lot.