well
theres always the risk in overclocking, that you can over do it, but i think you have to be doing something pretty wrong for that to happen.
is it worth it for the x1900xtx? well its pretty much the fastest money can buy, and can play any game you throw its way. i would say its probably not worth it. but if you want to have the absolute best performance, then you will have to overclock.
im not sure about overclocking shortening the life of a card, it probably will, more so if you play with voltages, but i think its not likely die in its lifetime with you (ie you will have moved on to something else)
if it was me, i wouldnt bother.