Personally, I'd probably go with the quad core G5 mac. The biggest reason is that we know it's stable. I'm not a huge fan of buying something so radically different as soon as it comes out. Even if the quad core intel is faster (which it may very well be), do you really need anything faster than a quad core G5?? Sure, rosetta works pretty well, but i'd rather run natively and not everything runs in rosetta anyway. Currently, I've got a 20" iMac G5. I bought it less than a year ago and of course, the intel iMac which is supposedly faster is already out. I'm not upset about it at all though. I LOVE my mac! It's faster than it needs to be and if you didn't know what was running inside, you'd probably never be able to tell.
As far as displays go, I'd go for the 30" ACD over dual 20" displays. Sure, you'll get more space with two 20's, but it's just not the same having two screens. I don't know if you've ever seen the 30" in person, but it's massive. I can't imagine needing more screen space. In my opinion, one 30" looks more impressive than 2 20" screens.
In response to your question about noise...macs are, in general, very quiet. The tower is louder than the iMac, but still very quiet compared to PC standards. Both the iMac and the PowerMacs use cooling zones with separate fans in each zone. These fans turn on and off (as well as speed up or slow down) based on the heat in a particular zone. Instead of running constantly on full power as they do in PC's, they only run when they're needed, at the speed they're needed. The zones are quite obvious if you open up any of the computers (the powermacs have 4 zones and 9 fans). Even when the fans are all being used, the computer remains quiet. When the computer isn't being used, it's damn near silent.
