If you are going for a standard, I would suggest that you dump x264 VfW and use the cli encoder instead. Check out MeGUI.
I have installed XviD v1.1 and using it's bitrate calc without actually encoding anything with xvid.
ztaticx check for both x264.exe (in the installer) and megui. In megui: Tools > Bitrate Calculator
x264 is the same as DivX & XviD bitrate-wise, so you can use the usual bitrate calculators you always used :)
... same goes for MKV ;)
yes... and less overhead means more bits for audio and video (read: more quality)