Lets see.. out of memory has only happened to me when I've used stuff with a known memory leak for a VERY long period of time (like 3 days or so).
While I'm not 100% sure, I'm guessing that Guild Wars does not have this problem, as I have left it on for weeks at a time.
This points to a hardware issue.
Is Guild Wars the only program you have that gives you the out of memory issue after extended use, or is it that Guild Wars is the only program you use extensivly?
I would say its not the system reqs. at all. As you said, you're waaay over their min.