Putting a computer together is typicially branded "hard"
It really is easy, OK, you could argue, that I have lots of experience with doing it, but with a little reading up, you can't go wrong. All the parts only go in one way, it's easy to match up certain slots with certain parts. It will be cheaper than having it built for you. If you really
wanted to, you could buy a DELL, although I would not recommend them for gaming.
The hardest part of building a computer, fully, with an operating system, has to be, installing the operating system, and that is easy enough.
I would strongly recommend you buy all the parts seperate, and build it yourself. Find a book on it at your local book shop, and read up, or, simply look at websites, such as this. And remember, if you needed any help, we are always here..
Here is a link to a very good website I just dug out, read up, and tell us what you think about building it up yourself: http://www.buildyourowncomputer.net/learntobuild.html
At the end of the day, you can go really slowly at it, you can take your time, there would be no rush (unless you are looking at getting on the computer really quickly)
You will find that with companies who offer to build your computer, they will take a while, and cost a bomb to do it. If you build it youself, you have the full guarantee, in which you would know which stage the computer is it, and what more needs doing. All the parts would come with a warranty, so no need to worry there. Also, you can gain great experience when building, gives you an in-sight into computers.