Can anybody give me some advice on the performance difference and/or capabilities of the NVIDIA GeForce 9400M + 9600M GT with 512MB on the 15" MBP and the ATI Radeon HD 4850 graphics with 512MB on the new 27" iMacs? I currently have a 3 year old black macbook and am somewhat torn between which of the two previously mentioned models to buy. I have a dell 24 inch monitor that is great that I could use externally on the MBP. Not as nice as the iMac screen. But still good. I know the iMac is more powerful, so I'm not looking on for comparisons on the system as a whole. Mainly just the difference/comparisons of the graphics cards. Thanks for any advice.