Good Advice!
I recently purchased a refurbished Dell at Computer Renaissance. Most of these units are new machines that were dead on arrival upon receipt of the original purchaser. Typically, the problems are minimal. The factory repairs them and sells them to resellers at hugely discounted wholesale prices. Although the warranty period may be shorter, it is the best "bang for your buck".
I purchased a Pentium 4 with 128 MB of RAM and a 20 GB hard drive for less than $270! It also came with a free refurbished Lexmark printer. (You will still need monitors and keyboards. They also have relatively good deals on these too.)
It's been about 1 year and I have not had a single problem.