Shown above, Photoshop CS5 is using 14GB memory all for itself, which means any system with 16GB memory would be feeling very sluggish— like an iMac, which maxes-out at 16GB. Or a MacBook pro, which maxes-out at 8GB. Or a MacBook Air, with a pathetic 4GB. All are nice, but all have serious limits, which is why a Mac Pro is the only good Mac investment for beefier jobs.
If you’re working with big files as I do, nothing less than 24GB memory will do (I use 48GB in a 12-core 3.33Ghz Mac Pro, because even 32GB won’t do on a few occasions). If I were using a 6-core Mac Pro for my main workstation, then I’d go with 32GB, even though it’s 2-3% slower in memory speed than 24GB, because I routinely push up against the limits of 24GB.
Apple won’t sell you more than 16GB of memory for the 4-core or 6-core Mac Pro, or 32GB for the 12-core Mac Pro. And Apple will charge your 4X as much per GB of memory. But with 8GB modules, those figures are doubled, and at less than 1/4 the cost per GB! View Mac Pro memory prices at OWC.
4/6-core Mac Pro: 32GB as 4 X 8GB or 48GB as 3 X 16GB
8/12-core Mac Pro: 64GB as 8 X 8GB or 96GB as 6 X 16GB
12GB vs 24GB
Skip 16GB, think in terms of either 12GB or 24GB (or 32GB or 48GB). If you think you need 16GB, go with 24GB, or go with 2 X 8GB (not 4 X 4GB) so that you can move to 24GB or 32GB later.
Most users won’t push the limits of 24GB memory, but lots of users might end up in a gray zone where 12GB is not quite enough, and that’s when big slowdowns start to diminish your productivity. Go with 24GB first, then if usage pushes the limits of 24GB, add one more 8GB module for a total of 32GB. See what’s going on with Activity Monitor. If need be, 4/6-core Mac Pro users can also go with 3 X 16GB for 48GB.