Sorry, but just because you can get 20-30% off of crap Dell boxes, dosent mean you're getting a deal.
Here are a few reasons.
Dell uses CRAP hardware, e.g.:

Cases that overheat
Intel Motherboards (cough)
Only Intel Chips (cough even more)
They bundle software, with 85% of it being software you will never use.
They flood your hdd with progs that carry spyware
And I am sure much more.

Sorry but Dell sucks. Their hardware sucks, and if you're a true techy you should always build your box from scratch, and never buy crap from Dell, Gateway or any crap like that.