Why should it be that BIOS reports 4GB memory, but System Monitor only reports 3.2GB? I've got two sticks of 1GB DDR2 5300/667 paired and one each of 5300/667 and 5400/667 paired.

The latter pairing may look odd, but I have the impression that 5400 just means that it may be run a little faster than 5300, so should match it.

(Ubuntu MATE 16.04 Beta)
Edit: 64 bit

Sounds like you are running 32-bit with no PAE enabled. With 4 GB I'd go 64-bit.

No - 64 bit, I've edited my first post to reflect that.

Some of your RAM may not be available if you use integrated graphics.

What's integrated graphics?

Post the output of:
Integrated graphics refers to a graphics adapter that is either on the motherboard or on the cpu as opposed to a discrete graphics card like from nvidia or ati. All intel graphics adapters are integrated graphics. Typically, your system bios will allocate a certain amount of your system ram memory for use by the integrated graphics. Discrete graphics adapters have their own memory on the card and do not use system ram.

1 members found this post helpful.

Thanks for a clearly explained answer. I found the onboard graphics inadequate for Ubuntu MATE, so I have installed a separate PCIE-1 graphics card with 128MB of memory.

chris@HPC:~$ free -m
total used free shared buff/cache available
Mem: 3260 974 1343 38 942 2194
Swap: 3999 0 3999
chris@HPC:~$

Depending on what motherboard you use it may be possible that memory remapping is either not supported on your system, or it is deactivated in the BIOS. Short explanation: PCI(e) peripherals need a place in memory to map their own RAM and I/O into, historically this is the part shortly before the 4GB milestone, in your case it seems that the motherboard reserved anything above 3.2GB for this mapping. This reserved memory area is not usable for other purposes, however, modern motherboards can map this inaccessible area above the 4GB milestone and with that make it usable again. For this to work two things are needed: 1. a motherboard that actually supports this procedure (which should basically be any mainboard built after 2010) and, 2. an OS that can address memory areas above 4GB.
Since 2. causing a problem is ruled out with using a 64 Bit OS it is likely that the cause for you is 1.

1 members found this post helpful.

Solved (Edit: should have written "explained" as it probably isn't fixable) here: https://plus.google.com/communities/...6658904?cfem=1.

Sorry, I somehow missed that post when I was making my previous one.
Clearly explained thanks. My motherboard is probably 2005ish so as you suggest, 1) is right. I expect that it is not possible/rather risky to update the chipset firmware and it's probably not worth it for 3/4GB anyway.

Just for laughs. Try memtest and see if it reports correctly the amount of ram. I assume there is really nothing wrong with your system and it's just one of those things.

Deleted.

I just get "command not found".

Generally you boot to grub and run memtest if installed or have to boot to a live media. You can't run it from within linux.

Almost every distro has it but you may wish to have this around.

http://www.ultimatebootcd.com/

I had checked what was reported on startup and I assumed that would be correct.

However I suppose that it could find memory but it could be defunct. I have now set the startup memtest to run and it reports the same amount.

Since I believed TobiSGD's diagnosis, I removed one 1GB card and replaced it with a 512MB card, with the result that the startup memory report was correct at 3.5GB and the OS saw 3.2GB as before. I am convinced that it is nothing to do with the memory itself (or the OS) and TobiSGD is right.