All I can say is, after installing MOS 3.10 (? I think it was 3.10...), that CPU 2 always runs about 20°C lower then CPU 1. Before both CPUs always had nearly same temperature (+/- 2°C)

To come back to Templarios question: In my opinion you can take any type of G5. It doesn't make a huge difference in every day use. Even Wayfarer isn't noticable slower on a1.8GHz (compared to 2.3GHz).
If you really need CPU-power for extensive application, take the 2.7GHz-version or 2.3GHz version to prevent all troubles with watercooling.
Be sure, that there is a supported grafic-card (Radeon!) built in. Take a look at MOS hardwarecompatibilitylist.