That's awesome. I was just going to post addressing the same issue
It seems that ATI Tool is actually a very poor artifact tester for current and next-gen hardware. Unfortunately, GPUs are more sensitive to core overclocking now because as you increase the GPU core speed, the greater the chance that you'll run into inaccuracies in pixel/vertex shaders. Unfortunately, ATI Tool seems to only use shader of version 1.1/1.4 when it scans for artifacts, unfortunately, games are using less and less 1.1/1.4 shaders and more 2.0+. What that means is that intense 2.0/3.0 shaders are going to show artifacting due to core overclocks much sooner than the you'd see texture artifacting or pipeline artifacting (dots, weird colors, etc). You'll see shader blipping, which is due to miscalculations in shaders from frame to frame. So what we really need is a tool that uses intensive geometry and shaders, this will cover the bases. Sadly, shader artifacting will occur LONG before we get a core lock or memory thrashing. For instance, my 6800GT will core lock at ~440, but will produce shader artifacting as low as 415 under harsh conditions (ie, Oblivion on a hot day). However, ATI Tool remains to be a good test for texture thrashing - caused by GDDR3 pushed to it's limit. ATI Tool is still good for this because of the texturing it uses - texture, diffuse, and a few other texture maps to produce the square, "fuzzy" box.
Overall, ATI Tool seems to be the best test for texture thrashing (memory), but 3DMark05/06 or Oblivion will probably be the best shader artifact tests for some time.