Just look at your maximum resolution on the card. Anything above 1920 x 1080 and you will be able to use a 24" at native resolution for most things. I believe that for films and office tasks the ability of the CPU and disk subsystem will be just as important and an 8800 with 320Mb is adequate for the task.
You can always run high end games at a lesser resolution equivalent to a 22" display or less. I would invest in the screen you want now then change the graphics if you feel it necessary later.
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The output resolution doesn't make much difference once you get to this sort of memory scale. 1920x1080 at 32 bits per pixel is ~8Mb, so even if an app/game/what-ever is triple-buffering the memory used by the frame buffers will only be ~24Mb.The extra memory will be used for thing like higher resolution textures (improving some visuals) or just being able to hold more texture data on the card saving trips to and from main memory (improving speed when moving between areas that might need different textures)
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1920 x 1080 x 32 = bits /8 = 8294400bytes /1024 = 8100kbytes /1024 = 7.91Mbytes
andy.