My one Prof. and I were talking about PCs a few weeks back. I was telling him about my custom built desktop, how nice it is, how much it cost, etc. He told me that if you know when the end of a term for Dell comes, you can get some of the best deals for prebuilt systems.
Basically, Dell supposedly has four terms year round (I believe the start of a term is when they release new systems that are higher priced with newer technology), and at the end of the term they start selling the old setups, only cheaper. What makes these deals even better is that these machines are built for you upon the time of order. All the parts are brand new, they haven't been sitting in storage for months. The PCs are guarenteed to work with all the parts in them and testing is done before they are shipped off to you. Plus you probably get a decent warranty as well.
These machines need to be ordered directly from Dell, since any Dell you buy at a store was probably sitting in storage.