A PC for web design and graphic design can be relatively basic. Pretty much any ‘off the shelf’ mid-range PC should be more than capable. Video editing is a bit different though, especially if you’re going to be working with 4K footage or doing some heavy after effects work.
I’m no PC master or anything, but I’d recommend a quad core i7, 16 to 32GB of RAM and two or more fast hard drives, SSD preferably. You can have a quad-octo-deca core i77 with 64TB of RAM, but you’re going to run into problems if you have just one 5,400rpm drive!
A top end graphics card isn’t that important for standard AE use, but you’ll want something decent if you’ll be using Element 3D or Premiere, and a lot of programs are making more use of the GPU these days. Usually best to go for nvidia. And if you have a decent card, then you can always play Black Ops 3 inbetween working, which I do. I’ll get in that winners circle one day, I’m sure of it. It’s for research purposes, honest.