I'm not sure if this is very helpful, but i don't think the error is the RAM. When RAM is faulty you tend to get a memory protection message (blue screen). And it tend to happen at random. Other games won't work fine, and then suddenly develop issues when running two instances of wow. Sure, it can be a load issue, but i think that Crysis would tax the ram more then two world of warcrafts running.

There might also be some issues with the RAM settings in the BIOS (you know the RAS, CAS and LAS thingies?). I would doubt the last one though.

I used to work in hardware support, and i did most RAM issues :-)