AFAICR, I've used 832x624 with SVGA monitors flat screen 15". The screen flickers a bit then locks on and works just fine. I never use 640 x 480 if I can avoid it.

I was using a Viewsonic or Sony VGA adapter with my Quadra 605.

In fact, I don't have an Apple monitor. All my monitors are VGA\SVGA\XGA PC types - mostly flat screens now. I can get 1024 x768 with my PM6500 and my G3 B&W.