WHy does my mac pro not access all of my installed memory?

What you have is failed defective RAM that you need to replace.

Try with just the 2 x 4GB in A1:2

Ideal would be replace the 4 x 512 with 2 x 2GB $54 which is what I use

http://www.amazon.com/BUFFERED-PC2-5300-FB-DIMM-APPLE-Memory/dp/B002ORUUAC/

It is not "accessing" it is not recognizing or seeing the RAM or its capacity.

Happens a lot with FBDIMMs, esp in warmer weater. More often DIMMs showing as half size.

And you do need to keep the fan speed default up higher to 800-900 rpm to keep FBDIMMs below 70*C.

you should not run with defective RAM. Take it out. Replace it, use RMA lifetime warranty.

You could pull the Riser and DIMMs and carefully inspect for dust and that they are seated properly and reset SMC, but I wouldn't waste my time or hold my breath.

4 or 8 DIMMs are ideal, quad sets even better (4 x 2GB for instance which would set you back $110 while you get this replaced).

Not as a resolution.

I want you to try with just one set, and that is where they should be.

you didn't say where and how but A1:2 is where highest density should be.

This is not a fix.

And the 512s should be on the lower Riser B

It could (lot of things "could" be)

Run Apple Hardware Test off your OEM DVD - it lists AHT on the label, and to hold down D on startup to run to test RAM and Risers

Also retry with just the 512MB alone, eiither on top riser or split A1: and B1:2

Risers are interchangeable and can go in top or bottom slot.

www.macupdate.com SmcFanControl 2.3+ launch and set profiles - it will be on your menu bar only.

RAM is in pairs and if one fails it could affect both or be the Riser. Without wasting time testing etc - as I've 'been there' with others and even if it appears okay today it will fail again and you don't want to run with flakey DIMMs.

Riser A has the 4 x 512MB now?

Air flow into the memory cage can be poor circulation.

And $110 for 8GB is the best price and quality I have seen.

you want to monitor sensors then use Hardware Monitor gives accurate. Temperature Monitor is freeware and lacks a few features but better than iStatPro or menu.

55C cannot be as each DIMM has its own sensor and there are many many more for other items, processor and cores, disk drives, each thermal zone.

Risers are A and B, there is no "C" it would be A3:4 or B3:4

Put all the 512s on bottom Riser. mix and stir. repeat.

Reset SMC after each change or zap pram.

check SMC's preferences and does it give you a choice of what goes on menu bar?

Temperature Monitor does give you menu bar with list of many - and you can reorder the list.

I had trouble with memory and buying over the years I took an interest was all and tried to brush up on it.

How to install:

https://support.apple.com/kb/HT4433

Mac Pro 2006:

https://support.apple.com/kb/SP30

symmetrical memory

https://support.apple.com/kb/TS1957

Sometimes people can't use the 4GB where you have them and when there are 512 or 1GB DIMMs, they work better 'by themselves' and always in lower numbered. High density should always be in top Riser, hence slots 1&2.

Mixing RAM does have some issues.

And some people thing 'pairs' go across Risers (one on A1, one on B1, etc) as if it was across memory banks - like in older PowerMacs at one time. Or more like the G5 even.

Might even want 1200 rpm to get down to 68C, until you decide to warrant these or something else. They should not though be hitting that high,, not with the fat RAM heat sinks.

at least you are one step closer now. And TM is accurate. For reference the FBDIMM I bought from Amazon run 60-65C with fans at default 499/599.