How To Install and Remove Memory Mac Pro2x2GB FBDIMM DDR2 667MHz @ $35
Over time, heat kills, and the stress of heat cycles.
I have heard/seen people lately have to go through 2-3 sets from OWC to get it working.
I would not even bother with 512's.
You should have quad set with a pair on each Riser. So $70 Amazon 8GB set for instance.
Always try iwth just new RAM installed. AHT and Rember is half way decent way to test.
Run the fans @ 800 rpm instead helps too. And monitor that they do stay in 65*C or less - more than 70 is not good.
The answer to your query has been presented to you. There is no point in attempting to test memory your computer can't see. You already have all the information you need. That memory is Bad.
Mac Pro 65lb tower features Error Correcting Code RAM. Eight additional syndrome bits are stored with each 64-bit word in main store. When read back, each and every memory transaction is checked by Hardware in the Xeon processor. Single-bit errors are corrected on the fly. Uncorrectable errors, such as double-bit errors are intended to cause a kernel panic.
But if your Mac Pro detects that a section of RAM is already Bad at Startup, it is simply mapped out. That is what has happened to you. The Power-On Self Test determined it was bad, and WILL NOT use it.
Hardware sat there and checked each transaction. It was Bad -- too bad to be usable. Accept that you have bad RAM and move on.
What may be really interesting is to check:
About this Mac > ( More Info ) Diagnostics ...
... to see if it left a report about that memory. If you find something, please post the message for the benefit of all readers.
There may also be information in the "Memory" pane, or it may simply say those slots are empty.