I had the same problem (needed a proper video card for an Inspiron 530S, though mine has a little more under the hood*). And I went with the Sapphire low-profile 4650, and am just thrilled to death with it. It was only $55 (http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16814102829),
runs off the installed 250W PSU without strain, runs cool and quiet, and eats up games. Installing it was a matter of downloading the latest drivers from the ATI site and popping the card in.
I can play L4D on high no problem. Also COD4 & 5, Mercs 2 on high at 1440x1050 and getting 30+ fps, etc.
* Mine has the Core 2 Duo E8400 w/ 3GBs. But read the reviews and see how it does on systems more like yours. (There're 91 reviews, and a majority of the purchasers seem to have been looking for a good card to fit a slim factor, off-the-shelf Dell, etc. And found it. But look for yourself.)