I didn't get to try these suggestions, but thanks a lot so far.
The DxDiag override should work, at least it does for my other OpenGL games since it just sets the fullscreen refresh rate, not depending on the fullscreen renderer. That's why I'll go with that registry entry first, I guess Freespace got its own set refresh rate.
I can't find the refresh rate setting in my video card drivers (8600 gts), I'm sure it was there in the past though. There's lots of application based settings, but no refresh rate.
Anyways, I'll play around with that registry entry later and tell you if it works.
edit: later, haha. The registry entry works like a charm, although I had to create it. Quite funny though, I did a little mistake and nothing worked, so I did a little searching and found an older thread here in the forum. Anyways, OGL_RefreshRate has to be created as a DWORD-value and my Freespace now perfectly runs in 75hz.
So, thanks The E, problem solved.
Thanks to Mongoose and FUBAR-BDHR as well of course, you did remind me of the option with the drivers, I should start looking where it went.