You can play around with all of the memory settings. Unless your system becomes unstable try to get as aggressive as you can. When I set a system up I'll tinker with the memory settings and loop 3dMark and Sandra for a few hours and see if the system crashes. If it doesn't crash, I'll set the memory a bit more aggressively and loop 3dMark and Sandra again. When it crashes you have likely reached your limit.
To start press any key. Where's the "any" key? --Homer Simpson.