I would advise less now and more when your son is older. The speed of development today [PC's - not your son] is amazing. Your needs can be satisfied easily on an average performance PC with a reasonable size HDD and plenty of memory. It will also handle childrens games! In 6 yrs when your lad wants play all the stuff all 10 yr olds want, your PC will be hopelessly outclassed and there will be the call for high end graphics and CPU performance - these come at a very high price. Have a look at the PCA Best Buy classes and you will get some idea.
Buying a "bare bones" high performance PC now, with a view to upgrading later will be buying built in obsolescence and in my view a bad investment - particularly with the expense of bring up a son who will want other things besides a PC for games!