Have you run Apple Hardware test? It should be on the original set of discs that came with your Mac.
Mac Pro 65lb towers use Error Correcting Code Memory, so simple memory errors are corrected on the fly, and uncorrectable memory errors cause a kernel panic to stop the computer before it poisons your data. Have you seen kernel panics?
Does that hardware test still live on the OS install DVD's
Only on the original-issue DVDs that shipped in the box with your new Mac Pro. The label will say something like, "to run Apple Hardware Test, Insert this disc and hold down the D key as you restart." If not labelled that way, it does not contain the diagnostic.
I've swapped out the Ram for a set I had just in case
The indicator of memory failure for MacPro is that memory will not be shown at its correct size (or not be shown at all) in System Profiler. Or you will get a kernel Panic.
Kernel Panics leave a log entry that can be read out later to determine the cause.