PowerMac 7200/90 Speedup
I really don't have much info on the 7200's, but there is a way to speed up the 7200/90's. The 90MHz version has a 20MHz crystal that generates the 90MHz signal that drives the PowerPC 601 at 90MHz. So there is a 4.5x multiplier in between the crystal and the processor. A 25MHz crystal will give a 112.5MHz machine. The max seems to be right around 120MHz, with a 26.67MHz oscillator. The modification is pretty straight forward; remove the 20MHz crystal (2 pins), and located at Y12, right behind the processor, and replace it with a faster crystal, or even better, a socket so you can try several different crystals. I have one report that the ethernet will go out when you run at 112.5MHz.
It seems there were two types of processors used in the 7200/90's:
The ones with a 601 have a second fan mounted to the side of the hard drive/power-supply enclosure (the thing that tips up to give you access to the logic board) and a Peltier cooler on the processor (there will be a cable going from the 'heat sink' on the processor to the logic board. These are pretty much at their max.
The rest have a 601+ which were made with a smaller mask and can be clock chipped reasonably well.