Yes it can be harmful, and it can reduce the lifetime of the card. However, unless you really get stupid you "probably" won't fry your GPU. As far as lifetime goes, as often as a gamer replaces/upgrades their video card you probably won't have any lifetime problems.
Think I have the same video card as you. I had OC'd mine when I was running Windows XP 64bit using Riva Tuner. Though I didn't see a huge difference and never OC'd again after I installed Vista. hehe I get good enough performance with most of the games I play that I haven't really messed with OC'ing it.