vax750 memory question

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We are currently interested another megabyte of main memory from DEC.
We need to know if anyone has had any experience with the two different
types of main memory available from DEC.
One is called "ecc/mos", the other is called "ecc/64k/mos". I realize
that one is more expensive than the other, but that is not the real issue
for us. We need to know if one is faster than the other, if one is more
reliable, etc.  I'm not sure what memory we have on our 750 right now,
so the other question is "does it matter whether we stay compatible?"
Thanks.
					Greg Hill