Do you mean a G4 PowerMac? or a G5 PowerMac?
Either of those have models with "lots of fans" (although the G5 typically would have more fans than any G4)
So, it would be helpful to know exactly which model. There's two distinctively different RAM types used in G5 PowerMacs
The labels on the RAM sticks will help you determine what type of memory you have.
Could be PC-2100, PC-2700, PC-3200 or PC2-4200.
The last two are, respectively for the two major different generations of G5 PowerMac.