What do I know ... but it might be the case that some contributing here might want to look at the Apple site and go through the 'configure your Mac' process. I could be wrong.
I'm confused -- your original question was,
>>"So how much does a graphics card have to do in PS for a one image at a time workflow? Am I on the right track?"<<
One image at a time in PS definitely presumes a SINGLE monitor and a 2D image; no multiple monitors and no video or 3D imaging. Moreover, your question, while not really clear, implies you are possibly wanting maximize Open GL features in CS4. Regardless, most of the answers I read are addressing your general question...
Answer me this: What good is a second video card going to do for a single monitor set up, when the stock video card can drive a pair of 30" displays?
Cheers,