3DMark as a test is pretty bad, use in-game benches to really see any differences in applicable graphics; in overclocking, you normally won't ever see HUGE increases in performance, it really is the goal of just pushing your specific hardware to maximum efficiency while remaining stable.
Try using ATITool as a tester to do some quick artifact scans, ideally, you can do it for a couple of hours to see if it won't artifact at your clock speeds.