My two cents
Arghh..I just tiped a very LONG reply, too long, at least my login expired before I ws ready, so here it's again in short form:
1. I realized that the possible refresh rate for a mode is dependant on screen size and position as set in the XF86config, which is the same you can adjust in SaX2.
2. If the saved or default size and position is out of certain bounds, the resolution cannot be displayed at all. For some reasons, with 3d acceleration activated, these bounds are somwhat tighter than without - at least I had a lot higher refresh rates before I activated 3d acceleration.
3. I could partially solve my refresh rate problem with the 1400x1050 mode in SaX2 by simply enlarging the screen (which raised the refresh rate from 75Hz to 85Hz). I had it at 100Hz before, but 85Hz is acceptable.
4. If a mode cannot be displayed maybe it can if the mode line in the XF86config is adjusted properly (but don't ask me how to do it "properly")
5. BTW: does anyone know the exact meanings of the mode line parameters?
6. Maybe a mode can also be "healed" by deleting the corresponding mode line in XF86config, but I'm not confident enough with Linux to recommend doing that. At least I know that a resolution can be displayed if there's no mode line featuring that mode. - If you delete a mode line and everything messes up, it's not my fault!
7. Why doesn't anyone answer to this thread who knows more than me 2-week-linux-experience-newbie??
8. good luck!