depends on what you want, for a good gaming computer you will pay out the butt, and it will most likely be outdated as soon as you get it. However building gets you the most bang for your buck. That being said a gaming computer will have the warranty and get fixed if it breaks, while a built one will not and if it breaks you have to fix it. Up to you however and if you know about computers it might be worth your while to build it yourself...just my humble opinion. However i plan on building one eventually.
This.
But it depends if you are just looking for a low end computer, then pre built computers aren't all that bad especially when paired with a monitor etc. Building your own boss machine is where Id always build your own. If you are just looking for a low end computer, Newegg(sign up for the emails) everyonce in a while has specials on some pretty badass (for the price and in context) "Bare-Bones" PC Kits that just come with the parts and you have to assemble it and provide a monitor, operating system, accessories etc. You can pick your own graphics card and put that in etc. Its usually a case/tower, power supply, motherboard, processor, ram & hard drive. Basically what you would get when buying a non gaming or video editing computer from the store - minus monitor, operating system and accessories.
Check out Anandtech, one of the best unbiased resources out there that I've always trusted since I was like 15(27 now). The build guides are pretty awesome and have components that will work together well. The only bad part is the build guides aren't done often so most of the time you have to look at it as just part of your research.
Its very time consuming to build your own "Quality" and customized to your needs pc if you have been out of the game. In technology things are always moving so fast that your year old computer while may by no means be a slouch, it is already getting blown away by the latest and greatest! In the time when I PC gamed and was really into computers growing up, it always made financial sense to get the step below the hottest thing on the market. It would save you hundreds of dollars per component and you'd still have a rocking machine!
http://www.anandtech.com/
Good Luck, lets us know what you decide!