Due to the age of the board, both cost effectiveness and a decent gaming PC with modern games is going to be very difficult. It uses an old socket AM2 processor which is not good for games any more, and DDR2 memory which costs more than the current DDR3 standard. You would be best served by adding $50 to the budget and get a motherboard like this one. http://www.newegg.com/Product/Product.aspx?Item=N82E16813128527
If you could let me know your total budget, I could fill in the rest.