Linden Lab's system requirements page gives minimum and recommended hardware for running their Second Life client for Windows, Mac OS X, and Linux in the form "so-and-so or better". The steady release of new graphics cards is a good thing, but it can make it hard to tell just what is better. (Graphics chip maker's nomenclature doesn't help.)
KirstenLee Cinquetti has made some interesting data available that may help. "Kirsten's GPU Awards 2011" picks out some graphics cards, both nVidia and ATI/AMD, as worthy of mention (and Intel graphics as particularly unworthy!), and has a pointer to a spreadsheet showing the summary data on which the awards were based: average fps and crash rates for various graphics hardware. One surprise: the poor showing of the nVidia 460, which I've always seen rated highly for price/performance. (A possible reason: the data doesn't split out the 768 MB 460s from the 1 GB; there are more differences than just the amount of RAM.)
Take a look. Alas, no indication is given of the graphics settings used, which makes it harder to compare... but I was utterly amazed by the crash rates shown! For example, KirstenLee calls the Radeon HD 4800 "stable", though its crash rate is 19.44%! Essentially one out of five runs crashes, and that's stable?! In comparison with the nVidia 270 at nearly 41%, perhaps. I'm not a gamer; is that kind of failure rate considered acceptable by gamers?