That depends on the application: Photoshop doesn't make very good use of the GPU (yet?), so you won't see any difference. CS4 accelerated only zooming, panning and rotation to my knowledge, CS5 adds a few more, but nothing substantial. Lightroom doesn't use the GPU at all.
On the other hand, apps like Aperture and Pixelmator can make much better use of a beefy GPU.
That depends on the application. The discrete graphics kicks in when certain functions are to be performed. If I were you, I wouldn't worry about it, the 320M is plenty powerful for most users. I would only insist on getting a machine with a beefy gpu if I knew what I'd be using it for.