The rule of thumb with many Macintoshes has been to install RAM chips in matched pairs for the best performance. This has changed with the new Nehalem-generation Mac Pro computers. You can install DIMMs in any configuration and the computer will work. But for best performance, populate the RAM slots in sets of three.
The new machines have their system memory directly connected to the processors through an integrated memory controller, which has three channels (with four slots for DIMMs) that are “hardwired” to each processor. For maximum RAM performance:
For a quad-core machine, use three DIMM chips (1GB and 2GB sizes are supported). Install three RAM chips in slots 1, 2, and 3, which loads the channels connected to the processor.
For an 8-core machine, use six DIMM chips (1GB, 2GB, and 4GB sizes are supported). Install three RAM chips in slots 1, 2, and 3, and also install three RAM chips in slots 5, 6, and 7, which loads the channels connected to the processor.
If you require more memory (and you’ve maxed out the first three slots for each processor), go ahead and fill slots 4 (quad-core) or slots 4 and 8 (8-core). There may be a slight drop in memory performance, but the tradeoff versus having access to the memory you need makes this negligible.
Further on the “plus” side, every time the computer restarts, system memory is checked for optimal performance. If the RAM set up is not optimal, the system will present options for improving the configuration.