Overdrive shouldnt be risky because it sets the clock speed on the fly depending on the temperature it reads. If you have good case cooling you can get higher then if you only have 1 fan and a small case with enough hot harddrives in there. But no, it's not risky and the improvment depends on the how much it OCs which depends on the temp. lol it's different for each environment.
It's not dangerious for the card, but just as a word of warning about overclocking other parts. If you're running another part excessively hot (only an OC could get it this hot, or bad ventilation, or being on top of a heat duct) then the extra 5-20 degrees that the card raises the case temp might be the breaking point on the OCed peice of hardware.
Nothing to be concerned with probably, but it just struck me when i was thinking about possible problems.
There is no risk, BUT it doesn't do a whole lot, at least according to virtually every review I've seen. It's more of a marketing thing/exhibition of ATI's future than anything else, IMO.
If the card is very cool in the first place, it can do alot and do it relativly safe. Doesnt do much on stock cooling though... If you are an eskimo you might be in luck