We’ll have more to say once the new version is announced, but very generally I can say
you’ll want at least 128MB RAM on a card that’s Shader Model 3.0 and OpenGL 2.0 compatible. Of course, more brawn is always welcome, and if you anticipate working with numerous large documents and/or 3D, having 512MB RAM on your card is a good idea.
For what it’s worth, I’ve been demoing by tossing around an enormous image on a 2-year-old ATI Radeon 1600 card (256MB) in a MacBook Pro, and it does just fine. If your GPU doesn’t meet Photoshop’s requirements, you won’t lose any features you have today, but certain new things won’t be enabled. As I say, we can get into more details soon.