I don't have anything planned at the moment -- too many other half completed projects to allow myself to start another -- but I'm interested in reading about what might be possible using affordable motherboards. When I complete my current G4 450MHz (18 months in progress so far...), I'll probably feel the need for something faster. I'm most interested in boards that are small enough to fit in a compact Mac or similar vintage case so the max size is roughly 8" by 11" (205mm by 280mm). A 1GHz or so G4 cpu would be nice plus AGP graphics.
G3 boards: They fit the size criterion but max processor speed is too low and I'd rather move on from PCI graphics on my best Mac.
G4 tower boards: Can anyone contribute to a definitive list of board sizes and psu connector types? I'm not afraid of modifying an ATX-style psu to work. G4 boards are very unusual on the second hand market here in the UK and complete donor systems are probably out of range at the moment. Early systems fail the AGP graphics card test and processor upgrades are expensive.
Cube boards: Potential donor systems are occasionally sold at the right sort of price (often cheaper than equivalent speed tower G4s in the UK). Suitable processor and video upgrades are possible for $$.
eMac boards: This is an unknown area for me. I can't determine the size of the motherboard and auxiliary boards from the disassembly photos I've seen. Can anyone point me to a comprehensive re-case hack that uses an eMac? As far as price goes, the eMac is very attractive because a new complete system from the education store is affordable and provides all the essential components. There's no need for third party accelerators to break the 1GHz barrier either.
PowerBook G4 boards: Too easy to break, too many issues (eg case interlocks), slower (might as well use a G3), limited expansion. Forget it.
G5 tower boards: Yes, when board size and money are not a problem... The G5 cube sounds like a better bet ;-).
Hard information rather than idle speculation, please -- I've contributed enough uninformed guff already in this thread. How about opening up that G4 in the corner and measuring the board for us all, eh?