virtualbox.org

It was 8 gig out of 16 which I would have thought as safe.

I saw the tech sheet that indicates the system hasn't hyperthreading. Strange that windows doesn't know that. (just checked task manager again and it shows

I wonder if there's some physical limit around 4G that I was hitting; 32 bit addressing somewhere?

As I explained, you have two 4 core non-hyperthreading CPUs which Windows would see as 8 cores.
There would be a 4GB limit if you were running a 32bit OS which you aren't.

Virtualbox freezes if I allocate more than 3.5M to the VM even though the system has 16G memory.
Also locks up if given more than half the cpu cores although I guess virtualbox can only handle one cpu, 4 cores.

Ubuntu 10.10 is out of support! Upgrade to 12.04 or 14.04