For graphic design and video editing you should primarily pay attention at two things: RAM size and CPU. These parameters are crucial.
The more RAM you get – the faster it work. If you want to guess how much memory you’ll need, do the following:
Run PS, corel & video editing software (in fact, all the software you can need to work at the same time). Open system monitor and see how much RAM is allocated. Multiply by 2. Add the size of biggest files you may need to work with. This will be aproximately your RAM size. Remember – better to take more than less.
As for the CPU, I’d recommend to choose Xeon, Dual or Quad-core.
Graphic card matters only when it comes to 3D modelling and gaming. So you can save on this.
As for hard drives, i agree with Vincent – you should have 2 physical drives (one for system and soft and other for work).
And the last thing: I strongly recommend to take a look at Mac Pro. I had a chance to compare MacPro 8-core with a PC of totally same configuration. It was the difference, really!
If want to do your creative work rather than fine-tuning your system tj become stable, buy a Mac.