Mac Pro not reporting memory amount correctly - please help

Question

Hi all,
I have recently purchased an additonal 2 x 4GB Crucial DDR2 FB-DIMM 667Mhz sticks for my Mac Pro Dual Core Intel Xeon 2 x 2.66GHz computer.  I have installed these correctly to Riser A 3+4 alongside 2 x 512MB Kingston sticks in Riser A 1+2, and 2 x 2GB Kingston sticks in Riser B 1+2.
I am seeing now that instead of the 13GB I should have showing in system profiler (which I initially saw after installation of the Crucial sticks), I am seeing 11GB.  The two 2GB Kingston sticks are both showing incorrectly as 1GB.
Can anyone help with regards what I should do to check if it's a Riser problem or the two sticks, etc...  I've ran a memory scan with TechTool Pro 5 and that came back fine, though for 11GB Ram and not the actual 13GB I should have/see.
I'm going to call Crucial and my local retailer tomorrow where I got the Kingston Ram pre-installed on purchase of the Mac, and that Kingston has a life-time warrenty anyway, but still I'd like to try and resolve this and look into the issue further myself too.
Please can someone kindly advise me what steps to take, I'd really appreciate it.
Cheers,
Ash

Answer 1

My advice, over the 5+ yrs Mac Pro memory:
Try with just the Crucial.
Remove the pair of 512s - doesn't add much
having 6 DIMMs is WORST config
best is 4 DIMMs matched quad set: 4 x 2GB or 4 x 4GB.
the smaller DIMMs should go down to 3/4.
Forget TTPro to test RAM. Use AHT yes but "no errors" usually means Riser is okay and mechanically the DIMMs are okay but might need more.
However, it is next to impossible to say for certain, and you can always find mixing brands or batches same can be a problem.
I would not say "correct" though.
correctly to Riser A 3+4 alongside 2 x 512MB Kingston sticks in Riser A 1+2, and 2 x 2GB Kingston sticks in Riser B 1+2.
High density or largest in DIMM slots 1 & 2
Sometimes a configuration works that doesn't make total sense, like all the 4 x 2GB all on Riser A.
Riser A: 2 x 4GB
Riser B: 2 x 2GB