Is 2 x 20W RMS good for a stereo system?

It depends on exactly what the "stereo system" actually is.

If it's a pair of high sensitivity horn-loaded speakers, being fed by a single-ended triode valve amp, then it's plenty.

If it's a cheap and nasty all-in-one system, then it will be almost inaudible, because the "claimed" output power of 2 x 20 watts will be far higher than the real output power.

Yes, it's "good", as long as the speakers are reasonably sensitive (at least 90 dB / 1 W / 1 m)

and you don't expect extremes of volume.

Doubling the power to 40 watts per channel would provide a minimal increase (3 dB),

just noticeable to most people.

200 watts per channel

(along with equally sensitive speakers

that could handle 200 watts each)

would provide double the volume possible from the 20 WPC system.

It's not vey powerful. I use an old Aiwa stero amp that I'd consider to me medium power - 60 watts per channel rms.

It's what it soubds like that matters though.