I don't know much about the Optiplex but I do have a Dimension 8200 which is an older model of the 8400. Anyway, my computer has served me well and I would recommend Dell. However, if I were to purchase a new comp from dell, I would buy a 4600 because they deliver pretty much the same performance as the 8400 for a lower price.
Our daughter is getting a new computer. As a student she can get a GX280 for less than the Dimension plus get longer warranty/support. Is there any area where the 280 will have less capability than the 8400?
Thanks

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