My computer has video performance problems. The sound is fine but the video is choppy. Sometimes it is so bad it is almost a succession of still pictures. The computer is a 400MHz Dell Dimension XPS R400 Pentium II with 512MB memory and Windows XP SP2. I just removed and reinstalled Windows, but this didn't help. My video card is 16MB STB nVidia TNT 3D AGP. Should I replace my video card?