OK, I finally sold out and got an apple (mac pro 2.6 w/3gb of ram) and to my dismay, all of my fears are being realized. I want to make motion run smoother, it's not quite as realtime as I had hoped, and "upgrading" a mac can be expensive and futile. I have "heard" that because motion uses a hearty amount of opengl and I just have the basic geforce that it could explain why my cpus are only running about 30% when it's playing like they should be running at 100%. However, I don't want to upgrade my video card only to find out that it doesn't matter at all, especially since I can't just go buy any video card, I have to buy the magic mac video card that has been blessed/price-gouged by steve jobs. None of the mac nerds can give me a straight answer, and it's becoming increasingly obvious that it is because they don't know what they are talking about. I usually get good advice on the cow, so I thought i'd see what you guys thought... ..also, is there a way to just buy some super kick ass bleeding edge gaming card and getting it to work with some open source drivers or something?
Motion is heavily dependent on the GPU on the graphics card for most of its magic. The VRAM on the card affects performance depending on the number of objects in the composite, and the CPU speed mostly affects things like particle system calculations.
More and more we will see the Video GPU playing a prominent role in pushing pixels around the screen. The Apple developers conference starts next week and we may see some new hardware announced (they just released the new MacBook Pros today)