I forgot to mention the expandability part. But I would guess that that is not much of a concern for this "build".
I would say unless the most speed is needed, Intel is the way to go. Lower power consumption, compact, OSX support for 10 more years?, modern ports, better GPUs and the Cores are not exactly wusses in the speed categories.
Actually, unless speed is an issue, I would get a MDD before a G5. The rest of the money can go for a SSD, top out in RAM, faster bigger "storage" drive, maybe a second LCD...........Hell some of them can even run OS9.