About G4 CPU uppgrade... rumors have it that there's a PPCcard in the making for the A1200 but so far it's just been rumors so don't hold your breath. If you want PPC go for a BlizzardPPC (me think G2).
About coldfire, although elbox is on this one and I do hope/think it'll release soon, is still only speculations of compability and it will most likely not work with existing hardware but need the dragon AGP/PCI bussboard (& towercase) to work.
If you're thinking about towers for modded A1200 then yes there are quite a few, else no. But original cases can be bought from Vesalia cheap so there's nothing stopping you from modding your own case from the original design if you want to keep the original just in case.
