Witcher 2 s a very CPU demanding game.So it will be very hard to come close to medium settings on your budget.Your also going to need to upgrade your PSU to support a better card.I would reccomend going with the 6770.
Do you have the i5 or the i3?
Sapphire Toxic 6770 $120=$100 after MIR + Free Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16814102937
Antec 430 watt $44 + Free Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16817371034&Tpk=antec%20430%20watt
With a 6770 you should come close to medium settings depending your resoultion and processor.I also suggest O.C.ing if you can.