Virtually any logic board can be shoe horned in any case. You just need some creativity and know-how. Not a good idea for a novice though. I can't see an MDD PSU fitting in a sawtooth case. The optical bay is too high compared to the MDD case. The main issue with an MDD board in a G5 or older G4 case would be you wouldn't be able to line up the pci cards with the case slots at all. The MDD was the only G4 with the slots on the top of the board. If you turned it aroun to have the slots at the bottom they would be facing the front of the case. You would have to find another way to stabilize the AGP and PCI cards. Would be bad for the card and slot to just have it leaning downward a bit without a screw to hold it.
To the OP:
The best way to not spend much and keep things simple is buy a NewerTech G4 1.6GHz upgrade for under 200 bucks. It's a 7447a CPU and you would end up with a system not far off the final PowerBook G4 which was a 1.67GHz 7447a. In simple terms it wouldn't be a screamer but certainly have enough pep to get things done at a reasonable clip. A single 7448 like I have would have a lot more speed for only about 150 more than the 7447 but saddly they only make dual 7448 upgrades now that cost almost 800.
The 1.6GHz 7447 is on sale now for $169:
http://eshop.macsales.com/item/Newer Technology/MAXG47S1600/
With that CPU and the RAM maxed out to 2GB you would have a very decent system.
I am kind of a Sawtooth expert since I own 3 so let me know if you want any other help.