Not got the 750, but have used the 800 and 850IS.
The 800 and 850 are about on par with regards to image quality.
The IS can be useful (esp. if you want non-flash pictures).
I can't remember off hand if the optical lengths are the same ranges.
all in all I'd say this is dependant on what you are likely to use the camera for.
If you only need it for holiday snaps or in daytime conditions then there isn't much of a point going to the 850IS, however if you're likely to want low light non flash pictures then the 850IS is probably worth getting.
Overall though I'd say try using the 750 (if it's already open), if you're happy with it don't spend more money for the sake of it.