I had Gentoo on a dual PIII 1ghz box. Ran perfect. It has since been reloaded with Win XP for all my video editing software, and TiVo ripping.

Also, I've got RedHat and Gentoo on a few other dual Celeron machines, and have never had a stability problem. I'd say the quality level is fairly good. I have noticed that the stock RedHat SMP kernels aren't as stable as they should be, but then again, who runs a stock kernel anymore?