Okay, I've currently got a total of 5GB in. Two 2GB sticks, and two 512K sticks. The 2GB are in Riser B, DIMM 1 and 2. The two 512K sticks are in Riser A DIMM 1 and 2. I've just ordered from OWC two 2GB sticks. So where do I install them, and do I need to move my original config. around to get the best performance? I've enclosed the current layout as it looks before the additional RAM arrives. Thanks for your help. It has been awhile since I had to revisit this and don't want to mess it up. I also have an ATI 3870 on the way. Maybe I ought to install that first and then tackle the memory thing after installing that so I can isolate any issues?