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THE 2600 PRO is using GDDR2 RAM which is considerably slower than the XT version which uses GDDR3. The architecture of the 2600 is that it thrives on memory bandwidth supplied by GDDR3. I would think 2600PRO and 9400M are evenly matched. favouring slight the Nvidia architecture with lesser number of ROPs. Check out Anandtech on their Graphics section where they did a 15 pages detail on the difference in architecture between ATI and Nvidia.
 
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