I have a PC with a NVIDIA Gefore 8600 with 256MB of ram. In the Windows (Vista) display properties, it would show 256MB dedicated, but 512 total (shared). If you are using Vista, go into the advanced Windows video properties and it will tell you that there is x amount of dedicated video memory and y amount of shared system memory. Does the iMac's 2600 Pro have the same feature? Can someone please check for me? Maybe attach a screen shot for me?