You may look at ebay or other online store who are selling cheap laptops because some are second hand units, but you should be observant in buying those kind of titems, you should know the specs of the unit and the warranty itself, and make a research on how long the unit will last.
I would suggets best buy or curcuit city, they may be a little pricier than others, but its worth it for your product, they do have some that are under $600
I recently bought a new Toughbook for a friend of mine who is on a boat and needed something that would resist saltwater. Found it on ebay and it was a good deal. He saved loads of money!
I am always nervous about buying things like that from Ebay... it seems like an important item to take a chance on! I have gotten some good deals from Overstock.com, though.
I LOVE overstock.com - but I would have a hard time buying a computer from someplace other than the company directly. I know that many of the PC companies run pretty good deals through out the year. With it tax time, they may have some going on now.
I think Ebay is okay if you really know what you're looking for, and have all the specs in front of you to compare. If you pay with paypal, you've got buy protection.