Most atx boards prior to SLI or ATI Crossfire boards have the 300w minimum for a supply just to run a completely basic system. The usual factory rec is 350w. This was the way it was until the Socket 754, 939, and newer P4 model boards arrived where the additional 12v feed was added. 400w and above is seen most of the time to cover SATA and newer video cards. If you buy a new supply from someone other then Dell you may to cut a small piece out of the case for it to fit. Dell has "their own" way of mounting supplies.
Someone here just recently cut a little 1.5" piece to see a better supply fit into a Dell case. That's Dell's way to make people buy supplies only from them. Thought you might want to know about that one. The card seen at Compusa is a newer model then seen at several online vendors where the Radeon 7000 and Radeon 9250 are the only others. That one is also newer then the NVidia models seen at those sites as well.