How fast mac do I need to make parallels run well?

Hi, I am running parallels now on a imac 2.0ghz with 4 gig of ram. It runs ok but bogs down with a graphics intensive program (Logos 4 Bible software) that I have on the Windows 7 side.

I am thinking of upgrading to a new machine and wonder how much speed is enough to make parallels run win7 well. I would prefer a macbook of some sort.

Thanks