I have a 2011 iMac in the house, and gave my parents a 2011 Macbook Air. All are on the most recent OS X. They all have more memory than 2GB, but does the new OS really take that much more memory? I've not checked.
I'm running El Capitan on a Mid-2009 Macbook Air with 2GB of RAM. It's not noticably worse than it was with prior OSX versions, might be a little better.
Should I try El Capitan first or go right to Sierra if I decide to update?