I had a 2900xt radeon now over 4 years old. it is dead.... lolz.! Well in catalyst drivers you get there is AUTO overclock which you can let the proggy do for you (that's what I did)
but generally you need to get an OC program and a GPU stress testing program and move up values 5-10mhz at a time after which you do stress test on GPU everytime to make sure it's not acting up weird, if with stress test the graphics get weird behavior/effects you know you went too far, and you should back off the mhz. that is the general guide line.
with old GPU you have to understand... they are old... what happened to mine is that the thermal grease had boiled over/dissipated from between chip and heat sink, and it self destructed in other words.
also..... SEARCH IN FORUMS FOR OVERCLOCKING GPU! there are tons of threads where you could get WAYYY better info. but i just gave you a warning
with a low end card like that you are not going to get any tangible benefits from overclocking....instead of putting the hours into it save your time and just buy a better card if you need more performance...at the most you get 10% better performance nothing you are gonna notice