its probably the NVidia Riva video card. I don't know for sure but the box says and i quote from the all mighty bottom of the box on this "NVidia GeForce series graphics processing unit with 32 MB or ATI Radeon 9000 graphics card (AGP)". that could be it right there. a Radeon 9600 will work perfectly.
Though I think you should get a Radeon 9600 XT . I'd get that i mean it was on the Tech TV top 20 gifts.