Note about the distribution not being the big bottleneck: This is exactly right.

On the other hand, if you use a distribution that somehow 'benchmarks' marginally better but you don't know how to configure it or deal with it properly, you'll end up losing any performance gains you otherwise would have had. Better to deal with a distribution that works the way you do (Which is why a lot of people like CentOS or Ubuntu Server) than end up making a major mistake somewhere because you are in unfamiliar territory.

If you're scaling horizontally, is 64 bit a win? It eats up memory in Ruby processes. We switched our app (which does not need to scale; it's shrinkwrap) to 32 bit Ubuntu and that was a win for us.

(Gentoo? Blech. -funroll-loops.)

We've noticed that several VPS providers are only offering 64-bit OS installs...which makes no sense. I kinda suspect it is simply a way to force people to pay for more memory down the road. The memory usage of dynamic languages (Ruby included) is dramatically higher in 64 bit builds...more than 50% in many cases.

If you don't have more than 4GB of RAM, it makes absolutely no sense to have a 64 bit OS install.

Depends on the app I guess, we do a lot of large number computations that surpass the 32-bit limit, so I think for our specific case a 64-bit distro is much better.