I have the first generation MDD DP 1.0 g4. I bought it when they were first announced, well within a few days anyways.
Let me start off by saying that I love the performance that I get out of this computer, and besides the drool factor of owning a new g5, this computer will suit me for a couple of years.
With that stated, let me say that this is the loudest computer that I've ever owned.
Depending on where you locate your computer (or what type of background noise you have in your working environment), it might not be an issue for you. But seeing how you are annoyed by the sound of a Rev. B iMac, I'm sure the sound will bother you as well.
Now I've been told that the second generation MDD computers had quieter fans, the noise level has still been found unacceptable by users. In fact there has been a website dedicated to the noise problem.
www.g4noise.com
I myself finally broke down and went with a solution that I found on this site, from a company in Germany called verax.
www.verax.de
Although somewhat pricey, I feel that in my case it will be worth the price ($300.00 US) if it brings the sound level down to where they claim.
http://www.verax.de/verax.php?SID=79683f8e140f72359&template=cms_entry_technik&groupid=1&entryid=27
There is even a posting from what seemed to me a very satisfied customer on the g4noise.com site.
http://www.g4noise.com/forum/index.php?act=ST&f=18&t=703
So depending on how much money you are willing to spend, or what you need/are looking for in a new computer, I'd keep this in mind and do a little more research.
Hope this helps