I don't think there's anything wrong with buying a pre-made gaming PC, so long as you do your research. I for one, don't like the idea personally, which is why I'm building my own. Mainly because I really like being in control of what exactly is being used in my build. pcpartpicker.com is a really great source to find parts. Essentially, you can make an infinite amount of builds with their database of parts, which the categorize by who sells it cheapest. You can also add custom parts and prices if you find something not on the site.
I built this one for myself:
http://pcpartpicker.com/user/GotCubes/saved/3Ixx
Though it's all up to do. It does however, require a lot of research if you're new to building a custom PC.
Have fun
~Cubes
Edit: My build is intentionally rather expensive. I've been saving for it, and I'm sure of the capable performance based the price. You can still get a good quality entry level gaming PC for $300-$400