Your hardware will likely depend on which s/w package you prefer to use. You mentioned Adobe After Effects so are you going the Adobe route?
Also, what format of video are you likely to edit?
Can all make a difference. I've had good experience with Matrox real time cards in the past so would consider them if you go Adobe.
As to the box itself...
Seperate HDD for OS and video projects.
64 bit Windows (if Windows is your thing)
All the RAM you can fit in it.
A couple fo decent monitors and a seperate TV for viewing your final output as it will be seen.