Without knowing your current power supply there is no way to recommend any card. Open the case and post what is written on the PSU sticker. It may be fine for a ~$200 card or it might not run an $80 card. The only way to know is ask your son ( if he knows ) or open the case.
cant believe forgot to get that down on my post
1.) Get your CPU Model
2.) Get your Power Supply Wattage/Manufacturer
You would need at LEAST 500-550 Watts to power the 5870. To comfortably use the 5870 I would make sure you have at least 600 Watts, I would go 650 depending on the price... If you don't have 550+ Watts in his computer, then you can get the ATI 5850 and couple it with 500 watts. Though even with the 5850 you would WANT 600. I like to give my power supply room to breath if my budget allows. With power supplies, the more wattage, the better (As long as it is from a reputable company ie: Corsiar).