Your first statement was pretty close, you can use a RGB-VGA adaptor to use your system on a VGA, using a DblPAL or DblNTSC screenmode, but as you say old games / programs will not usually let you select a Dbl mode and will thus be invisible.
Old Amiga monitors (the Philips 8833II for instance) will show everything but flicker in hi res modes.
Scandoublers - hard to find and expensive, but display everything rock solid. Aparrently some are better colour-wise than others (see separate threads.)
