eh, i'd take the extra money you'd spend on a quad core and put it towards a 2nd monitor
and a tablet....if you're going to be doing this a lot you'll thank yourself for having that 2nd monitor....
and just make sure you go w/nvidia for a graphics card shake works much better w/nvidia....
i also wonder what type of footage you plan on compositing with? film? HD? DV?
if you're going to be comping film or HD size stuff be prepared for your HD to fill up quickly