My vps has 768MB guaranteed, burst to 1.5GB - WHM/cpanel
At the moment we are using suPHP & suEXEC with no opcode cache at all.
I had the hosting provider enable fastCGI & APC set up for opcode cache, and after enabling those it lasted a few minutes the ceremoniously crashed... so we are back to suPHP which does not utilize APC.
So I am curious if APC / fastCGI is possible with a small amount of memory. I've not asked for any tips from my hosting provider yet.