thanks for all the information and help.
unfortunately i tried out the xtreme-g ones intensively now and was somehow disappointed with the results. my test game (oblivion) seemed not just to look exactly the same as before, it also seemed to be a bit slower. i made a testrun with 3dmark and got a 4679 score - with pretty much overclocked settings - that made me wonder. my previous scores, without any overclocking at all, were always above 5000.
so i removed the xtreme-g drivers and reinstalled the original nvidia 84.21 ones - 3dmark is up at 5331 again WITHOUT any overclocking, and oblivion is displaying my usual framerates.
i don't want to talk bad about the xtreme-g drivers (i loved the new control panel btw!) but for me they don't seem to work, sorry
btw, currently i'm using the old ntune version 2 utility for overclocking and that works ok for me, so i think i'm gonna stick with it for now. thanks for the help anyway!