one good way to get a good deal with Dell is to check their outlet. Usually they post refurbs, scratch and dent, ordered but never used, and customer return systems there at GOOD prices.
I got an Inspiron 531 with an AMD dual core 4200+ (that rates to about 6GHz on the Intel scale because of the dual core- ie each processor is roughly 3GHz) with a 320 GB HD, 2GB of Ram, DVD-RW, and Vista Premium for under $400. It was listed as scratch and dent, but had never been used. It came with one scratch wayyyy down on the bottom of the case where the panel slides on and off (not noticeable unless you're taking said panel on or off). Otherwise, it was in PERFECT condition. I added another GB of Ram to it for less than $50 and for about $450 have a system that is more than adequate for my needs.
I did add a video card to it, as most of the Dell systems on the site stock (except for the XPS) come with integrated video and those aren't really good for much of anything. Grand total for system plus all upgrades was still under $500 even after shipping.