We need your computer specs! Power Supply, processor and screen resolution especially.
I would suggest a Radeon HD 4850 (a couple years old but great for the money), except I don't know if your computer can handle it. I think a 4850 can max SC2 at 1080p, but as with all RTS games, processor speed is very important too. The best part is, a 4850 1GB can be found for only $80 after rebate on newegg.comhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814102834&cm_re=4850-_-14-102-834-_-Product