Linden Lab have been collecting data on Second Life viewer frame-rates and have compiled them into a handy little chart - Okay, so it's a handy big chart. The chart shows the 100 most widely used GPUs, approximate frame rate ranges and medians.
Of course some of this data will vary a little. People with more capable hardware tend to crank up the settings until things start to visibly slow down, so we expect the higher-end hardware to show a bit lower as people flip on a lot of extra rendering options and crank up their draw-distances.
Remember in Second Life, all content is downloaded on the fly instead of being preinstalled on your hard-drive (that's why the Second Life viewer is only 35MB or so), and time spent receiving network data is not time spent drawing graphics frames - so lower frame-rates are to be expected. You can have quite a smooth Second Life experience in the 10fps-20fps range, and the system is quite usable down to about 5fps.
If you're shopping for a new video card though, to pump up your Second Life graphics - the chart is definitely worth a look.