Running several applications involving PHP on virtual private server - optimization possible?

I currently have 1 domain on a VPS with 1 instance of Moodle 2.3.2. To
minimize the danger of overtaxing the resources (I share a processor :-(
but have 2GBytes of guaranteed RAM) I was wondering if there were anything
I could do if I wanted to put, say, Wordpress or another domain on the same
server with another instance of Moodle in terms of PHP tuning. For the
moment all is well, but I prefer to avoid problems in future if possible.
Ah yes, my PHP version is 5.3.3.