For your needs, something like HD 5450 would be perfect:http://www.newegg.com/Product/Product.aspx?Item=N82E16814131339
Though, could you be more clear about the restarting part? Maybe just not graphics card got damaged?
To upgrade a hard drive, it's simple: pick out how much space you need or how much you want to spend. Generally, 1 TB drives have the best space/price ratio.