There are several AO722 models which come with 4GB RAM, so I'm pretty sure it's theoretically possible.http://us.acer.com/ac/en/US/content/models/aspireone722
You could take off the hood of the memory compartment, if you're comfortable doing such things (and you're sure you're not violating your warranty terms) to see if there's an empty second memory slot.
If you have one 2GB memory stick, you should try to buy the same size and if possible, the same make and model too. This increases the chance that the computer will be able to use them in "dual channel" mode, which actually gets things slightly faster.
Anyway, have you checked the memory consumption of the running processes? Is it really the case, that you're out of memory? (I kind of doubt that, unless you run something which eats it up... sometimes browsers can do that... or some process running in the background, like Windows search indexer, or Windows Update, or virus scan...)
It seems that I was trying to be too clever above :Phttp://www.youtube.com/watch?v=aryiM71keOE
There's only one memory slot. If you want more memory, you'll have to buy a 4GB one, and swap.
What a strange shape that memory module has. Is that the DDR3 standard for netbooks? Or is it Acer-specific?