It's my custom to order our Mac towers with minimal memory. Apple's memory is of good quality but I feel they charge too much. So I vote with my dollars. I have found reliable alternate sources for Mac Pro memory.
I look for companies that have a reputation for providing strong upgrade support to Mac power users. Two good examples are Trans International and Other World Computing.
With the help of samples of 1GB, 2GB and 4GB FB-DIMMs from both Other World Computing and Trans International, we were able to test 12 different memory configurations using our special 64-bit parallel multi-threaded version of STREAM to see which of those configurations produced the best memory bandwidth.
In the Table below, we averaged the results for the Copy, Scale, Add, and Triad functions of STREAM to produce a single speed rating in gigabytes per second. Test system was an "early 2008" Mac Pro 3.2GHz 8-core system running Mac OS X Leopard 10.5.2.
FILL ALL 8 SLOTS, GO FASTER
As you can see from the results posted above, any configuration that filled all 8 memory slots produced the 7.5GB/s average speed. However, the advantage of the 8 slot configurations over the slowest was only 15%. You may feel you can live with that until you can afford to fill all 8 slots.
THE "X" FACTOR
Though some of our configurations involved various size mixtures, we consulted with an engineer knowledgeable about the architecture of the "early 2008" Mac Pro. He suggested two things that may not show up in our benchmarking:
1. The pairs on the top riser (slots 1&2 or slots 3&4) should match the pairs in the bottom riser (slots 1&2 or slots 3&4) in terms of size if you want the maximum benefit to be gained by interleaving in real world applications.
2. The 2GB and 4GB FB-DIMMs are typically Dual-Rank (DR) while the 1GB FB-DIMMs tend to be Single-Rank. Ideally, you want all DR modules.
We ran a stress test created by Lloyd Chambers of DigLloyd.com to heat up the memory. It uses all the memory and runs the 8 cores at 100%. During the stress test, the temperature, according to Temperature Monitor, averaged 160 degrees F for both OWC and TransIntl memory. Of course, each module reported a different temperature with the highest being 176 deg F in both cases. No modules came near 190 deg F, at which they supposedly "break a sweat."
HOW MUCH MEMORY IS ENOUGH?
"You can never be too thin, too rich, or have too much memory."
How much memory you need depends on what you do and how you do it. As I write this, I have 10 relatively mundane user apps running on my MacBook Pro (Mail, Safari, Address Book, Calendar, Preview, etc.). Activity Monitor reports 2.67GB in use. I'm glad I have 4GB installed.
We ran the Retouch Artists benchmark action file using 300MB file in Adobe Photoshop CS3. We observed how much memory was gobbled up during the run. It was the only app running, yet, though we specified only 3GB of memory cache, Activity Monitor reported 13+GB in use! What's happening is that Mac OS X Leopard has the ability to hand over unused memory to apps for caching when they use up their 3.5GB allocation limit. (Ditto for Tiger.)
When we render the TotalBenchmark with After Effects CS3 with Multiprocessing enabled, it spawns 8 sub processes on our 8-core system which together grab 13+ GB out of our total of 16GB.
So how much memory should you buy for your Mac Pro? As much as you can afford. We recommend 8GB for most Mac Pro users (8 x 1GB or 4 x 2GB). We have both of our 8-core Mac Pros configured with 16 GB (8 x 2GB). If we could afford it, we'd run 32GB (8 x 4GB) in both.
SPECIAL THANKS TO...
John Poole at Primate Labs (creators of Geekbench) built a special 64-bit and parallel multi-threaded (openmp) version of STREAM 5.8 for us.