My answer (others will argue, your mileage may vary) is don't do it. It is an iffy proposition to begin with, and will wear out the equipment faster in the long run. If you are conservative with your settings, you will likely do no harm, but you won't see a worth-while increase in performance. If you go to the edges, well, bring money...and you might or might not see significant improvements, and then only in some games, at some points, in some resolutions, depending on the phase of the moon on Hoth…
If you like playing with such things (I actually do, but never on my production machines), be extremely careful, start low and slow (and start with the CPU, not the video card) and listen to people (cruise the blogs and gaming websites) before you try something. Good luck.