There was a bigger Xserve G5 based cluster built, but it belongs to a Military contractor so I guess supercomputer lists are not high on their priority list.
The next monster computers will surely be running the Cell I would think. Its based on PPC like the G5, so should be good for the same sort of tasks, but now we are talking 9 cores per chip. And I believe I saw a PCI-E card which included several Cells for rendering purposes, so I guess you can squeeze a whole lotta power into a small space, which thrashes the hell out of the overheads (System X had to build a building for it as I recall), especially the aircon bill.