🚀 go-pugleaf

Besides the scan doubler by Princeton, there is the Sigma Designs
Color400 board which does scan doubling AND true 640x400.  Sigma
Designs is providing drivers for VDI and MS Windows.  There are
companies making Color400 clones.  So maybe this board will be
a "standard" like Hercules.  Although Sigma Designs just
announced their own EGA clone, they claim they will continue to 
support the Color400.

If you are going to sell software, one of the EGA clones with
with EGA, CGA, MDA, and Hercules modes seems a better bet.  But
beware that the CGA mode works better in some EGA clones than
in others.

I have a Sigma Designs Color400 and the SR-12 for an AT.  I have
been quite satisfied with this combination.  The ROM of the Color400
does have a bug that freezes the cursor indicator when using
Crosstalk on an AT.  Sigma Designs has promised a fix in a few weeks.

If I was buying today, I would probably get the NEC Multisync
monitor with one of the EGA clones (one that worked right!)
trading off a slightly lower resolution for maximum flexiblity
(PGA clones or higher resolutions are just around the corner and
the Multisync should be able to handle them).