[DJG3]

Just ordered 4x 4g sticks from crucial, however only 2 arrived and the rest are on backorder. Currently have 4x 512s with 2 installed on each riser.

Originally was going to just pull the 512s and just run the new sticks to make 16g which should be a solid improvement for now, then pick up another 2 or 4 sticks later on. However, since I have no idea when they'll be back in stock should I just run both 4g sticks I have now together on the first riser, or put one in each riser? Or I could run both 4g sticks on riser a, then 2x 512s in the remaining slots in riser a, and 2 of them on riser b while im waiting for the rest of my ram to arrive. Or just put 1 4g chip in each riser and wait it out with 8g? Not that 8 would be all that bad for the time being. Thoughts...

[Simon]

On the original 2006 Mac Pro you can populate six slots. I'd put your 4x512MB on one riser card and the 2x4GB you already have on the first two slots of the other riser (make sure to use the lowest-numbered slots).

Here's Apple's memory expansion guide for the original Mac Pro. Note, they show an example with 6 populated slots.

4 or 8 DIMMs are better than 2 or 6 from a memory bandwidth POV, but since you have four of the 512MB DIMMs (single-rank) they'll make up for most of the difference to the double-rank 4 GB DIMMs. There's a whole lot of information about bandwidth and performance on the original Mac Pro here.

[AKcrab]

Why didn't apple follow their own guidelines for 2 DIMMs? When you get a new machine, there is one DIMM on riser A and one DIMM on riser B.

[Simon]

Originally Posted by AKcrab 

Why didn't apple follow their own guidelines for 2 DIMMs? When you get a new machine, there is one DIMM on riser A and one DIMM on riser B.

I think you might be mistaken. The instructions I linked to above are for the original 2006 Mac Pro since that's what the OP has.

However, the last-generation Mac Pro (Early 2008) had a different memory bus architecture. For those Mac Pros you were supposed to install the first two DIMMs across both risers. That's why Apple shipped the stock model with one DIMM on each riser. You'll find memory expansion instructions for the 2008 Mac Pro here.

It's important not to mix different MP generations up as instructions on how to expand memory are entirely different.