ibuypower is pretty good if you want to pick your stuff but don't feel confident about building yourself
they have decent prices, they are very good about their warranties and they build a solid system. I know quite a few people that have bought from them and they have all been happy. I have also helped people do upgrades and repairs on older machines from them and they are very well put together, not what you'd expect from a prebuilt.
Building yourself will be cheaper, but its just slander to call something a rip off just because they are charging you for a completely ready to use machine rather than just parts. They have a Warranty, tech support as well as labor and other costs to think of. These are solid machines and at a pretty good price for what you are getting.