There is no amount of money that would ever turn this into a serious gaming rig. You would be better off building a new system for that.
However, for Internet, server, storage etc. the only thing missing is the storage part. You could simply add something like this. http://www.newegg.com/Product/Product.aspx?Item=N82E16822136110
An external HDD might also be a good option since you have USB 2.0.
As stated above, your case will fit a standard uATX MB, so it could be used in the upgrade. However, it would need some slight modification like maybe a side fan.