Well, you could use CS4 with 8GB ram on a Mac?
And aren't Macs usually the prefered platform for graphics etc?
Not without getting, to be blunt, the shaft, with Ram prices or Apples stupid product line up. If I want a quad core machine with 8 gig of ram - it's little short of £2k. That would buy me a whole new workstation with 8 gig of ram, a proper workstation system disk ( velociraptor or SSD), plenty of storage, plus three quad core, 4gb render nodes.
And as for Graphics - I'm OS ambivolent. imho, I find all the CS4 apps run better on XP than OSX. Truthfully - they're all fairly crap in the CS4 incarnation - Adobe have dropped the ball in a major way - but they work better for me, on XP.