VIDEO CARD PROBLem

AGP Video Card problem.
I have a question. I just purchased a ATI radeon 9550 AGP video card 256DDR. The problem is when I try to play GTA or Halo, the video is terrible. I mean by that: missing graphics on the screen (i.e. there should be people or cars and nothing is on the screen, dragging of graphics across the screen, freezing of screen.
I have a Pentium III 833mhz. Win 2K as operating system. 392mb ram and 80gig harddrive.

Any suggestions?