hi,
i had a GTS about 8 months ago and i posted the same question, and as nobody answered i had to find out for myself.
anyway,
most GTS cards will do you to 220mhz on the core ,at least
the memory however is a mixed bag, depending on both the "ns" rating of the chips and the cooling that they have.
i invested in two sets of thermaltake DDR ramsinks and attached them to both sides of my GTS (creative). and the best i could get was 350mhz with no artifacts. i was hoping for at least 400mhz to compare with the PRO cards but i couldnt get it there.
I heard of some GTS card reaching 400mhz but they are rare. if you want some info on overclockig your card specificly, then look on the net for a review of your card in particular. then just look at the overclocking section of the review.
hope that helps