For Video editing you need at least a machine with high CPU power. If you do it professioal or as “professional” hobby you should get a new board with a fast CPU.
With Adobe Premiere or After Effects a Core 2Quad should be supported since this are professioal tools. You should get at least 4GB RAM and should think about a 64 bit OS. I’m not sure if there are 64 bit versions of Premiere or After Effects.
If you’d get an Intel system you should look for a board with P35 or X38 Chipset. As CPU a C2Q Q9550, if you’d spend more money get a QX9650. The QX9770 is too expensife IMO. If you think a C2D is good enough get a E8400 or a E8500.
At least you need 2 fast HDDs. I’d get 2 or 3 good and fast SATA drives. MAybe a RAID0 is a good idea for faster HDD access during video encoding?
At least I’d assume a price for a good video editing system around 1200-1500â‚¬. If you spend this money you’ll have a really fast machine for your video stuff.