Yikes uses a PCI graphics card, Sawtooth uses an AGP graphics card. Just open it up, if the graphics card is plugged into a brown socket, it is AGP, if it is white, it is PCI.
If it is a Sawtooth, it uses a special (not standard) socket for the processor (which is on a daughter card), it can easily be replaced with a top of the line G4 chip for a few hundred. The Yikes can be upgraded as well, but I not sure as to their limits, as they use a standard ZIF socket for the connection.
Go to xlr8yourmac.com for all the guides and info you want.
TEG