I got the version of Riven from GoG, and I'm having a few issues.
1.) Turning off 640x480 mode doesn't actually give me any scaling. All it does is allow the adapter to maintain its normal resolution instead of switching to 640 mode, but the graphics remain steadfastly 640x480, a tiny little window in the middle of the screen. Is there any way to get the images and movies to scale to fit the screen?
2.) After a few minutes play, it crashes, just locks up.
I have it set to Windows XP compatibility mode.

http://www.gog.com/en/forum/myst_series/riven_freezes_often this should give you your answer for the freezing... though i don't know anything about the scaling. I also have windows 7 premium 32 bit and i just play with the small window. try searching the rest of the threads in the past to see if anyone else had that problem.