Are you using it just for Video Editing or are you doing 3D graphics work as well ??
You will need to look at the minimum and recommended specs for the software you are using and make a decision really. It is hard to give a clear cut answer to these questions, as it would also depend on your budget. If you have £10000 spend then go for the biggest baddest you can get but if you only have £500 then you might be limiting yourself.
Minimum (basic usage) hardware
32-bit dual core 2Ghz CPU with SSE2 support.
2 GB RAM.
24 bits 1280×768 display.
Mouse or trackpad.
OpenGL 2.1 compatible graphics with 512 MB RAM.
This is quite a low requirement but the more complex work you do it will take longer to render, so getting more Ram, CPU and GPU will make things run quicker.
A Quad core CPU
8+ GB ram
A decent GPU (2gb+)
The above specs are pretty much standard for most basic gaming PC's but this is where I would start, you will need to decide on how much work you are going to do and what kind of work as 3D modeling and rendering can take a while to render depending on the complexities you are doing.