20 inch model has only 256 MB graphics card and 2.66 processor. $1500
For DVD authoring, Mpeg Streamclip, Snapz Pro, tube TV it will be more than adequate.
24 inch model bumps to 2.88 processor, 500 GB internal, and 512 graphics card. $2000
Faster transcoding, (but really how much faster?) Full Final Cut Studio Compatibility (including Motion 3). May be better for Second Life due to graphics card, but I will have to research.
24 inch pimped out. But it costs $2500.
3.06 processor, 4 GB RAM, 750 internal HD. FCP editing and all things the Mac Pro can do.