Okay Ill try to help you out a little here...
First off, curious of why you need 8 USB Ports? You dont want to try to edit off of 8 different hard drives. Trust me, its extremely slow. Using 3 externals at once is annoying and slow.
I actually still use my macbook pro sometimes which is the same specs as your macbook to do a lot of my editing and graphics. It works perfectly fine. That being said, I do understand why you want/need more out of your machine.
What are you shooting with to convert to ProRes 4:2:2? Unless you are working with a RED or something of that level, then your data rates and size will not be bigger then DVC Pro HD. The AVC HD with ProRes runs at the same data rate as DVC Pro HD. The RED at 4k 24p comes in at about 225 if your wondering.
I can run FCP and AE on this machine at the same time (although its a little sketch, but thats also adding in I hardly have any room on my hd). 8 gigs of ram really wont be necessary either if your doing stuff for editing. 8 gigs and up is pretty much for people doing 3d animation like animated movies.
4 or 6 gigs will do you just fine. Any of the new stock processors will work just as well too. Quad Core or 8 Core will do the trick.
Im not too knowledgeable on graphics cards, but I would suggest an HD one.
Make sure you do somehow get a nice screen. Your monitor will be key for Color Correcting and you wont regret forking out some cash for a nice HD monitor.
Here is what I am running: (and its more then I need even running AE, FCP, Color, Magic Bullet Looks, and other small programs at once)
3 gig processor with 8 cores
6 gig memory
4x ATI Radeon HD 2600 XT Graphics cards (you only need one I believe)
23" Cinema HD Display
1x 500 gig internal drive
That is all more then you would need. The set up cost me under 4, which might be out of the range your looking for, but I also bought this over 2 years ago, so it should be cheaper now. Plus if you going to college you can get an insane student discount with these things if you buy it through apple.
As for what E Heath said, I would strongly suggest that set up instead. The only thing about the iMacs is that they pretty much are as they come. What I mean by that is they are not easy to upgrade and add anything. The Mac Pros you can add/upgrade graphics cards, add memory, add displays or get new ones, add internal hard drives, change out processors, ect. Which is really why people get them. Could save you some money 5 years down the road.
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