No idea about the placement, but I have read here and elsewhere that, if you've got four sticks, you want to populate an entire memory riser board, rather than putting two on one riser and two on another.
The diagram on the inside cover of my Mac Pro shows the following fill order -
1 - two back slots on top riser
2 - two back slots on bottom riser
3 - two front slots on top riser
4 - two front slots on bottom riser
If you dig thru Apple's support forums for the Mac Pro you'll find a couple 0f posts where someone actually tested several differerent combos.
As I recall consensus was to first determine target amount of ram then work toward that using only the back rows of the two risers.
Other combos work and essentially are very useable for the majority of users.
But using that approach should yield the best performance for those doing things like 3D rendering under Rosetta.
In that case - where each frame can take six to fifteen hours ti render and you have 7200 frames to do then a 3% speed increase is definitelty desirable.
For readers who might not have bought extra ram yet it's also important to get quality, reliable, stable ram.
Although the worst seems to be past I've read reports of buyers getting flakey ram from Crucial , OWC house brand (but not the TechWorks they sell - although there may have been some problems with it as well) and even Apple.
It's advisable to stress the system after adding ram by running a tough memory diagnostic test at leat overnight - 24 or 48 hours duration is better. Some problems don't show up right away. A 48 hour stress test will usually find any sticks that are marginal and might be prone to early failure.
bfx
