Honestly, just check besybuy, newegg, tigerdirect and whatever retailer comes up through google. That of course if you are only looking to spend $600-900.
I would personally just find someone you know to build one for $100. You could find a build on the forums or ask for help, then just get the person to build it for you. All the major parts will come with 2-3 year warranty at least, while most prebuilts in that range only have 1 year warranty.
You get a better quality of parts when a build is done custom. In a prebuilts the motherboard and power supply will be VERY bad. If you paid someone to build it, it would cost about the same as buying a prebuilt.
As mentioned, building a computer is rather easy, which is why so many kids do it
It shouldn't take someone who has done it before more than 2 hours for all the installation and updates.
Here is a site if you want to learn the process about building a computer (not saying you are building it, but someone else building it for you even)
http://www.build-gaming-computers.com/
Video of a full build, 30 minutes
https://www.youtube.com/watch?v=w-uHmL7Y2UU