Fubaar,
As goates said, it will work with a 4X AGP Slot but it is not much of a Video Card. I do not recommend that card to anyone. It just does not have the horsepower to run the new games currently being released. If your son is really into games, then look at a GeForceFX 5700 Ultra or an ATI All in Wonder 9600Pro. Those two cards are currently in the Best Bang for the Buck range.
The GeForceFX uses up to 75W of power when under load. It does take a good power supply to run a GeForceFX or Radeon system with either an AthlonXP or a PIV.
I thought the A7A was an ALI Chipset for the Athlon, not the PIII. Are you sure the motherboard is the A7A?
Take Care,
Richard