How to stress memory on web server?

I have created web cluster with several web sites and I would like to perform stress test of a memory but I have no clue how to.... I have used Jmeter but I always get CPU load not memory (cant used more then 1GB). Is there anyway to stress test it by Jmeter?

p.s. I would like to avoid custom scripts because I want my test to be repeatable by anyone. For websites I am using generic WP installations.