Have you tried running the game at stock speeds? I have my GT conservatively overclocked to 400/1.9 (stock cooling) and I get no artifacting in FarCry or Doom 3, but in HL2 it happens sometimes. HL2 runs better than either of the former two, but that doesn't mean the card isn't being pushed. Also, why haven't you updated to nvidia's official 66.93 drivers? Until you update your drivers and try the game at stock speeds, I don't know what else to say.