It uses non ECC memory so your assumption is off base...
Is the new RAM 1066 ? ?
And each slot only supports so much.
That being stated that'sn about the worst written User Manual i've seen in 5 years...
The new RAM is the same specs as the Kingston RAM that is in it except for the Non ECC part.
From what all I've been able to find some workstation PCs will only use the Registered 72bit RAM.
*copy paste cause I'm lazy*
I've found a post on the Acer forum from a tech that says there only 7 chips that they guarantee will work and he only listed 3 of those in the response.
So found a listing on Ebay for an identical stick as the one in it (one of the ones listed by the Acer tech)and bought it.
I'll finally get the 4 gigs I was going for at least, just hate buying used memory.
And YES the Service Guide as well as the Owner's Manual are just about as vague and unhelpful as I've ever seen.