a 720 is at best equivalent to the igp and at worst is less performing.
i'd look at used or open boxed cards to try and maximize that budget rather than wasting the money on something that is barely an upgrade.
here is a gtx 740 refurbished that is better than anything new i see below $70
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500384 it is $60 and will offer an actual upgrade to your current igp.