The only one with Liquid Cooling is the Early 2005 Dual 2.7 GHz. All the other ones are Large Heat Sync with Fan Arrays.
I love the design of the G5, it's some of the Jony Ive's best work. I have a Dual 1.8 Ghz collecting dust (waiting for Morhos), and a Quad 2.5 GHz running Kubuntu 12.04.