XO memory size

[Derek Zhou]

Hi all,
I am running debxo on a XO I got from 2008 G1G1. I just noticed that the memory size is only 
221776K according to /proc/meminfo. I know that there is 16M used as video memory, so there should be 256M-16M = 240M available to linux, right? I search around and see some people has about 237M. What does it should be and what else is occupying memory? I am on the latest firmware Q2e32; before that I was running Q2e22 and they all report the same. I forgot what I got from the original software that bundled with the machine, though.
Another thing is X drawing is very slow; however if I add:
Option "FBSize" "8388608"
to xorg.conf, it becomes visibly faster. Why is limiting the video ram to half the size make it faster? 
Thanks in advance
Derek