Help with a Inspiron 530s (slim case)

I'm looking into upgrading my Inspiron 530s for better gaming (I've been told that it was a bad choice of computers, especially for gaming, but it was cheap and I can't take it back now). I've been trying to decide between a few graphics cards the best low profile cards I have found are the Sparkle GeForce 9800 GT (http://www.newegg.com/Product/Product.aspx?Item=N82E16814187061), the Sapphire Radeon HD 4550 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814102819), and the Sapphire Radeon HD 4650 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814102819). I don't know a lot about computers and such so help in this department would be nice. I've also heard that I need a larger power supply than the 530s' standard 250w, but I’m sure the slim case would pose a problem there. Thanks for any help.