it is always safe to overclock as there are always a certain amount of overhead built into the chips manufactured. the best route for a good reliable overclock is to overclock the video card by a small amount to either the core or the memory (one at a time) and then run a iteration of 3dmark or play a very intensive (low framerates) game for about 15min.
what you will be looking for in these games is texture corruptions and artifacts. corruptions are easy to spot and you'll know one when you see it, artifacts are a bit different, it will look like "snow" small glitches only a few pixels big that pop up on the screen. the harder you overclock your card the more of these two things you'll see.
if your 9500 is on a red PCB I would try looking into a soft-mod for it to mod it into a 9700 non-pro.
yes overclocking does reduce the life of your video card, but by the time it dies if you did a safe overclock and treat the card well. . . I'm pretty sure you'll be dead too
