In that case, it could be a bottleneck. Phenom x4 955 is quite powerful and 4250 is, well, not
You should upgrade according to how much gaming you/your son plan to do. If it's only SC2, a 6770 will do just great and it's pretty cheap.http://www.newegg.com/Product/Product.aspx?Item=N82E16814121474
However, the gpu might still bottleneck the cpu. I'm not sure, but in any case the game will run a lot better.