8gb flakey ram PROBLEM

Question

I just got a Mac Pro 2.66. Decided that 1gb ram wasn't enough. After checking around message boards decided to go with transintl.com.
Called up at 4:17 on a Thursday afternoon. Nice guy Mike said go ahead and order and he will make sure they go over night. Ordered 4x 2gb sticks total 8gb ram. Got them the next day. So far I am happiness.
I put the first pair in riser A first two slots. Next pair went into riser B first two slots. Fired up the MP and all looked well. I took a look at "About This Mac" and it said 4gb. I clicked on "More Info" and then on Diagnostics and it said DIMMS in riser B slot 2 failed. I thought alrighty must have not seated the RAM fully. Pulled and reinserted all RAM. Booted and checked "About This Mac" it said 8gb. OK problem solved.
Today, just for the heck of it, I checked again and it said 4gb with the same problem in riser B. I ran "Apple Hardware Test" from the CD, choose the extended tests and waited about 48 minutes. All passed all clear. I rebooted and checked again and I now have 8gb. @$#%.
Should I send back the two sticks?
Is there something else I should be checking?
How do I know if it is the RAM or the MP?
How reliable is the hardware test?
I am new to MAC and really want this machine to work out so please help.
Mac Pro 4 core 2.66   Mac OS X (10.4.10)   2x 750gb 7.2k + 150 gb 10k 8gb (4x2gb)

Answer 1

By the way are the actual Riser boards specific to Slots A and B? In other words can I plug the whole Riser A board with the good DIMMS into Riser B and vice versa?
The risers are physically the same so in that regard you can simply put either riser is either slot. The only difference being the RAM that's on each riser.
Is that the best way to stick them both in Riser A as opposed to one in A one in B?
It is always best to populate Riser A before Riser B.
I never really see anyone selling odd number RAM configurations. Is there a penalty for having the extra gig (2x512gb) in one Riser and not the other?
Technically yes. By not matching Risers A and B you can have a situation where the RAM difference is not operating at the full, 256-bit wide, bandwidth. Be aware that this ONLY applies to the difference between the Risers so in your case it'll be only the 1GB.
Is it enough to justify buying another 2 512gb memory sticks and put them into Riser B for a nice 10GB package?
Ultimately your milage will vary depending on how much and how often you saturate yuor RAM. In your case, any performance "loss" will result only when you saturate your RAM by more than 8GB. If you do that a lot then you might want to consider going to 10GB but I probably wouldn't bother. Regardless of any potential performance hit, having 9GB, presuming you need it, is always better than having 8GB as RAM operating at 128-bit is better than having to resort to hard disk (virtual memory).
In reality the difference in performance won't be much even if you fully saturate your RAM. You will gain more if you actually need that 10th GB.