pk18 :
As far as the gaming my son would need to use it when I'm not on the computer. I have another desktop that plays his games pretty good. Its an HP w/512MB Nvidia 84or 8500 Ge Force. Something simular to this.
Ok-- well then you just have to decide how much you are willing to spend for your son, because the Geforce 8400 or 8500 cards are MUCH less powerful than even the $50 card on the "best graphics card for the money" article I just posted. If you look on the last page of it you will see a hierarchy that compares cards.
I see two obvious paths for you:
1. Choose a card that can run on your current power supply. These cards would include an ATI 4670($65-70), Nvidia 9600GT($80) or GT 240 with GDDR5 ($80). These cards wouldn't let your son run new games on the maximum possible settings, but he would be able to play new games with the graphics turned probably 80% of the way up and anything more than a year or so old just fine. (this is dependent on monitor size though) World of Warcraft and massively multiplayer online games like that would run fine on a card like this.
2. Get a new power supply to run a better card. This has some other benefits as well--your system will be quieter, more power efficient and more reliable. (I bought my father a ZT Affinity and I know that power supply fan is LOUD and I cant even hear the corsair 400cx power supply thats in my computer running)
If you decide to get a new power supply you will have a variety of options. My personal suggestions:
Corsair 400w(39.99 afer MIR) + ATI 4850 graphic card ($99) =$140+ shipping (newegg doesnt have tax)
Corsair 400w(39.99 afer MIR) + ATI 5770 graphic card ($155)= $195+ shipping
Of these two cards the 5770 is newer, more powerful, and more energy efficient, however I really think the 4850 would be enough to keep your kid satisfied--and is a very good value at $100. (its much more powerful than the cards I previously listed)
Many of the graphic card debates on this website are between adults that are hardcore gamers that spend thousands on their computers--you should keep this in mind when reading opinions that tell you to buy $200 cards.
If you want more advice on the card to get you should list what games your son does and or wants to play, and the size of the monitor. This would help make the determination more clear.