3dmark03, as much flak as it is taking, has been my most recent choice. the most sensitive test(visually) to artifacts in my experience has been the scene where the chick enters the library above the ogre hideout. an unstable overclock will show a degraded backdrop (white specs over the blackness). therefore, it is not necessary to run the whole benchmark.
so, this will tell you at what speed artifacts become visually obvious, but it doesn't mean that the card will crash during games, so sacrificing visual quality for speed might be an option.
good luck