I've done this before, but for some reason I can't figure it out again.
I want to specify to use positive sync for horizontal and vertical in
the XF86Config file... Reason is that it makes it much faster when I
switch between a MS Windows machine with a KVM. XFree in linux wants to
use negative by default, but my windows machine uses positive.

I remember doing this without doing a full modeline. I just put +VSync
and +HSync somewhere, and it worked..