I can tell you what I have done.
About 2,5 years ago I decided to take YouTube data creation seriously and I wanted a system on which I can do video editing and create screencasts, my main focus is Linux Tutorials but it can be anything.
My big problem was that I only had like $300 - $400 to start with.
So I found a shop that buys and sell computers from big business which buys computers on bulk about once every four years and then after the four years is finished and they buy new computers they sell those old computers to people and companies. So I bought my notebook there.
The main things which I wanted was an i5 or i7 CPU and 8gb of ram, the rest I can upgrade later on.
I found an HP Elitebook 8460p which was in my budget. I later upgraded the ram and the hard drive to an SSD later on. I made already about $2300 on youtube so it paid off the price I paid for the notebook.
So the things I would look at now if I will buy a new notebook or desktop. Don't go new as older is still great and a lot cheaper. If you go with a notebook then get something in which you can replace the ram and hard drive easily. Get a monitor with at least 1920 x 1080 resolution and i5 CPU is the minimum!
I hope it helps!