From BareFeats:
http://www.barefeats.com/quad09.html
"DUAL CHANNEL vs QUAD CHANNEL
In order to get the full benefit of the Mac Pro's 256 bit memory data path, you'll want to populate both memory riser cards, each with at least one matched pair. If you put your memory on only one riser, you are dropping from quad channel to dual channel mode. See Apple's Mac Pro memory notes for more this.
Does this translate into faster real world speed? Not always. Though the Xbench memory fill rate test showed a 34% gain, it doesn't necessarily translate to faster application speeds. We ran some typical tests from our suite of real world tests (iMovie render effect, Final Cut Pro render clip, Cinebench CPU render, Motion render RAM preview, iMaginator Core Image morph). None of them showed any gains from Quad Channel mode.
Anandtech did some testing with real world apps using dual and quad channel memory configurations, too. Of their 15 real world tests, only 2 showed any gains from Quad channel configuration.
We originally posted some gains from iDVD encoding and UT2004 Botmatches but later tests contradicted our results. We're in the process of retesting. Until we're sure there's a real world gain, we pulled those graphs.
HOW MUCH MEMORY SHOULD YOU BUY?
My good friend, Lloyd Chambers, has done a lot of experimentation in this area. I recommend you read an article on his blog that addresses this question, especially for Photoshop users.
Lloyd feels that anything over 8GB is wasted. Yet, in the case of active multiple applications, each of which can use over 3GB, I say you can never be too thin, too rich or have too much memory. We also observed that, when we were just using Photoshop CS2 to edit a 1GB sample, OS X (Tiger) used the remaining memory as a disk cache, saving us hits on the scratch disk.
What's the least you should get? I say no less than 2GB. Even if you are a gamer that runs one thing at a time, apps like World of Warcraft want over 1GB just for themselves. Add what OS X wants, and you are already over 1.5GB. You do NOT want to be constantly doing virtually memory hits."
From diglloyd:
http://diglloyd.com/diglloyd/free/MacPro/memory.html
