I know mwave.com, censuspc.com, directron.com does the select the parts and then you can have them put it together or you put it together. I'd say the best thing to do is go to fatwallet.com or slickdeals.com and when they have cheap power supply and case combo, buy them. You see all those other parts, hard drive, dvd, ram, motherboard, cpu flunk down and up all the time, but cases and powersupply are less common. If you take this advice, you can build a computer for $200-300 but you yourself have to put it together, you can do it, watch youtube, go to google, there are people offering free instructions.
If you put it together through the other sites I've listed, its safe, but ends up costing you more, and you would be better off going to dell.com.