I was going to recommend the 640mb version if your going to be playing games on a large monitor and with high resolutions...
after some reading it seems that the 320mb dosent fall to short of the 640.
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they say in the conculsion:
The lack of 320Mb on the cheaper GTS only becomes noticeable when you run your games at extremely high resolutions with very high AA levels. If you’re running at 1600x1200 or more modest 1280x1024 resolutions you’d have to look very hard to find a performance difference.