When it comes to image quality, talk to anyone other than devotees of Olympus, who will not hear a word spoken against their cameras, and many will tell you that a lot more depends on their quality control (which was at one time notoriously wobbly) than on the lens design.
Originally Posted by BBonte
Certainly, when I had both f/2 and f/1.2 Nikkors -- a stop and a half, not just the 2/3 stop difference you are talking about -- I didn't find that the extra brightness made a great difference, though the shallower depth of field it did make focusing easier.
To get a good idea of how much difference it would make to brightness, stop your f/1.8 down a little beyond f/2 and use the preview button.
Personally I wouldn't bother -- though a 50/1.4 would probably be so cheap nowadays that it wouldn't cost much to find out.
If you want bright viewing, consider a rangefinder -- or even a 50mm finder in the accessory shoe of a reflex.