OS X Lion behaving oddly after upgrading to 16GB RAM
I got the "System is low on disk space/memory" error as well as a number of other intermittent problems when I upgraded my original apple memory.  It turned out to be either bad memory, or marginal timing specs.

Startup disk has no more space available for application memory?

When I put the original memory back in, the problems went away - so try that if you've exhausted your other options.

I did eventually do a little more research and found a 16GB set that very closely matched Apple specs for my early 2011 macbook pro 17 memory, and it's had no problems for several months now.

I suggest trying out the original apple memory and seeing if the problem goes away.  If it does, return the memory you have, and see if you can get a different set that works better.