[Hampshire] How to tell how much memory?

Hi all, hope the barbie is going/went well ;-)

Here's a funny thing.
Acer box from a reputable source; 1 GB memory ordered, 1 GB on the
delivery note, 1GB on the label on the front of the PC.
BIOS says Total memory 1048576K.
On booting up, the initial screen (which only shows for a couple of
seconds) says Memory Testing... 104xxxxx OK. (didn't have time to get
it fully.)
I have just had the side off the box for cleaning and it appears to be
fitted with two identical memory modules each labelled 512 MB.

But ...
memtest says it is testing 768 MB.
'top' says 769500 K.
System Monitor 751.5 MB.
/proc/meminfo 769500 .
dmesg : 762904K/786368K total.

All I can think of is that one of my mem sticks is faulty and thinks
it is 512 MB, and reports this, but in fact has 'lost' half of it
(address line stuck?)

Any clues? I think the m/c may still be under warranty, under 1 yr
old, but I'm reluctant to send it away and anyway I had to peel off
the warranty sticker to take the side off. (It really did need a clean
up inside; I live/work in a rather dusty old-house environment and
there are building works in progress.)

cheers all,

victor