Putting active air cooling on a 2.7GHz G5 probably isn't a very good idea.
The reason they used liquid cooling wasn't because of the power. Intel was using fans at the time and they ran up to 150W (IIRC the G5s topped out around 130W).

The problem was that the G5, being RISC is a physically smaller chip. It is significantly smaller than the AMD or Intel parts so has to dissipate heat over a smaller area.