Thanks to Catmoran's website http://www.catmoran.com/olpc/#sugxterm
, I found the answer to your question about finding which build is on the XO:
Current OS version
To find the software version, press ctrl + alt + [Friends key f2]. You should see something like OLPC Build 650 on a black screen. To get back to the desktop, press ctrl + alt + [Home key f3]
My first XO from December came with build 650, the one for my daughter that arrived last week came with 656. I was wondering what the big difference was and if it was worth upgrading to build 656. The only difference I have seen so far is that build 650 goes to sleep after 20 minutes and a touch of a button brings it back up. This is convenient when I am reading an ebook that does not link to each chapter. The build 656 one seemed to turn itself off and then I had to scroll to my place in the ebook.
But, I am gettng ready to take the plunge to go into terminal for the first time and install flash and mplayer. If I am going to do that, I may as well upgrade the build 650 one first from what Grandma wrote.
Does anybody know of any other advantages to build 656?