Many of the heatsinks are not interchangable without some "hacking" being involved. Also I found out that the air-cooled heatsinks are usually not good enough for the liquid-cooled models. I tried using 1.8ghz heatsinks on my quad. Although they did mount together, there was no way to secure them to the case so I had to run the machine on it's side. I think the Quad CPU was too hot and the 1.8 heatsink was insufficient since it only had 4 heatpipes. I believe the dual-core models had 6 heat pipes but those heatsinks are different.
Now I'm sure you can somehow shoehorn some other types of heatsinks with more heatpipes onto the G5 CPU but that will take some workmanship to pull it off in order to mount them. I have a thread up on this if you want to search through my history.