If its video editing for after, with a mind to move towards after effects.
While the CPU is important to this, Really you should look more at the RAM your going to use and how much (lots is the answer), and IMO the most important thing, the GPU.
In a windows i7 machine, I run 16GB of DDR3 1600MHz ram, and use a PNY Nvidia Quadro FX card. (the 4800 if your interested)
This made the biggest difference to the editing suite. The difference between gaming cards and professional editing cards is immense. The pro cards do things like effects on the fly, and rendering on the fly, and these are the bits that really make multiple layers of 1080p possible. And I have a friend who runs the 3800 in his, and again this works fantastically.
And as well as the GPU you can start to look at addional cards, we've been using the matrox CompressHD card for a while. And absolutely love it.
Also you should look at hard drive arrays. An SSD for the boot drive and to run the programs from. Then a scratch drive, for after effects, plus a media storage drive, plus a project drive, plus a backup drive. (which if you stick the SSD in the second optical drive bay it all just abouts fits)
But yeah, the other point to note out is that a good few of the plug-ins that we use for Adobe Premiere and after effects are only single processor enabled. so 20 cores at 2.2Ghz isnt going to be as fast as 2 cores at 3.5Ghz. (for those effects at least)
Video effects, and especially at 1080p are going to be huge, and really test out a system.