Fun fact: VGA *is* RGB. You can literally just hook the wires up directly, though it may be a bit dim (a video amplifier would fix this).
It's difficult to troubleshoot with a potentially active device in the way, but it sounds like you didn't get soft15khz working correctly. The double image effect is normal when trying to display progressive scan 480-line video on a standard res monitor. Soft15khz is very picky about what version of the driver you're using due to how it works (it's basically an automated registry hacker). Ensure you're using not only a known compatible card but also a known compatible driver version.
You want 640x480, but it needs to be INTERLACED. Alternately, you could also run something like 320x240 progressive. When properly "installed", soft15khz will make the 640x480 option in Windows output interlaced video rather than progressive (the default, for compatibility with normal PC monitors which cannot handle 15kHz scanrates).