What kind of performance do you need?/how much are you willing to spend?
If you are just looking for basic video watching performance a 4550 is a good choice.
The best performing low profile card for you would probably be an ATI 5570 around $80.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102874&cm_re=5570-_-14-102-874-_-Product
Power use of competition in a test system:
(the 9600GT/9800GT are the most powerful low profile options but probably wouldnt work on your PSU)