Im no expert on computers and have had the same problem as you. I can only suggest that you check to see if your motherboard supports your new graphics card. ie it complies with your AGP specifications 4X, 2X AGP modes. Or try checking your directx settings to see if that is working ok. I have had to go back to my old graphics card (geforce2 mx400)to be able to play games. If i come across any other info I'l let you know.
Hope it helps