If you have a slim tower then you need a low profile (or low profile ready) graphic card. Within your budget there's 2 cards i'd recommend, both are HD4650s.
This one is low profile ready, which means you'd have to replace the backplate with one that's included in the package.http://www.newegg.com/Product/Product.aspx?Item=N82E16814131151
this other is an actual low profile card.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102829
Here's a graphic card hierarchy chart so you can see where the HD4650 DDR2 is listed.http://www.tomshardware.com/reviews/best-graphics-card,2362-6.html
Neither cards needs to be plugged into your power supply.