The idea is: buy the most modern, not a more faster old one... will be still a old one. Go for the Quad or if you want to expend a little more get the new i7... more cores, more faster.
About the video card we will start a little war: I have a lot of people with problems with ATI cards, so if you dont want trouble go for a NVidia card, but you dont need to have a Quadro. In a conversation with friends someone said that to use a Quadro with all feattures Carrara need a new drive so both can talk...so a FX, but more new them your old 7600 is the way out.

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