The Ultimate PPC approach is pretty much the most 'reasonable' approach you can do (if there is such a reason).
Anything "fast" from today's POV (>>1 GHz) will get extremely slowed down by the rest of the system (SCSI, I/O, graphics). The minimum a fast CPU requires is fast memory.
Then there's the slow bus, so you better add a current one as well. Alternatively, you add everything required anyway directly to the board, saving you the hassle of dealing with various drivers required: USB, SATA, graphics.
After a while that leaves you with the old system just used for keyboard and power. Worth it?
However, I'd love to see a decent Ultimate PPC board getting released. Maybe it's even affordable.
OTOH I'd really love to see a PCIe card with a Minimig/Natami on it to run on "alternative systems". With chiplevel emulation/simulation the look and feel would be perfect without all the implications of running ancient hardware. (Don't get me wrong, I still love my 3000 for what it is but find it hard to invest hundreds or even thousands of Euros into it.)
