The normal average fan speed for the PowerMac G5 Quad should be in between 1800-2000 rpm, so yes your Quad fans are working at normal speed. As long as it doesn't get so loud like the 8bit guy's loaned Quad (browse Youtube to hear that loud fan noise), then your Quad is fine. Now, that is a good example of a Quad that is leaking and fan going at full tilt.
Both the Quad G5 (late 2005) are as reliable as the G5 air cooled version, except you have an unknown factor to deal with, that is the liquid in the cooling aren't meant to last forever. At least with my air cooled G5, I can dismantle the CPUs, repaste them and then put them back together and recalibrate the fan. I don't have to deal with the liquid factor and I came to the conclusion that for music work, I don't really need a Quad. And for brute force compute work with video, I have the Mac Pro 5,1 that can deal with that. So that's why I opted to adopt the air cooled G5.