Personally, 20FPS is just too uncomfortable to play. 30FPS can be marginally playable if the framerate is stable, but if it's jumping up and down and just averaging 30, it's still too uncomfortable. At around 40-45FPS is where I find most games to be playable. For a long time I used budget or midrange cards and had that kind of performance. Now I have a somewhat more powerful card and usually run 60+ FPS. I would prefer not to go back to 45FPS, let alone 30 or 20.
...another side-effect of PC gaming. Once you've experience high frame rate gaming, you can "never" turn back.