The new Dells are actually really quiet. =P I have a Zalman CNPS3000 in "silent" mode on my proc, several panaflos running at 5V in my case, and a Seagate Barracuda IV. Cool as a cucumber and very quiet.
So as far a specs go, pretty much whatever you want will be fine. The main noisy parts in the comp are the processor fan, graphics card fan, hard drive, and case fans. Zalman makes a good VGA cooler that works on high end graphics cards, so you can get a cutting edge machine that's nearly silent.