well, those in the screen shot are definatly artifacts, even if they are in a grid. they are not mean to be in the ati tool display, and they have been identified by ati tool as artifacts.
if you dont see them in games, then i wouldnt worry though, to be honest.
to check more, 3dm06's "deep freeze" will locate any problems with the video memory, and google for "rthdribl" and that will pull up any problems as well.
hope that helps.