You shouldn't be artifacting. There are a couple of possibilities:
1a. You overclocked the card and the overclock is not stable, turn down your overclock and try again. If you wish to maintain your current overclock, you'll need to bump up your voltage a bit.
1b. Your card is not overclocked and is still artifacting, in which case you may have a defective card.
2. This is a driver problem, try switching to an earlier driver revision and see if the problem persists. If it does not, then Crysis Warhead simply doesn't agree with your current driver revision for some reason.