If you're on Ruby 1.9.1 and want to use Rails 3 Beta 4, you have to upgrade to 1.9.2 or the new Rails beta won't work.
I have not looked at the Ruby side in a while. I know the 1.8 series put Ruby as one of the slowest of the dynamic language but vaguely remember that being worked on. Had 1.9.X brought a good speed improvement?
There are a lot of projects aiming to fix that, but yes, YARV is a great improvement on the speed front. Synthetic benchmarks being what they are, it looks promising: http://rubychan.de/share/yarv_speedups.html
Ruby 1.8 uses MRI (Matz's Ruby Interpreter), which is indeed slow. Ruby 1.9 throws out the old MRI code entirely in favor of YARV (Yet Another Ruby VM), aka KRI (Koichi's Ruby Interpreter), which is considerably faster.
How well does JRuby stack up against them?
I have been coding a new app with 1.9.2-head and the latest Rails 3 beta. RVM makes switching from 1.8.6 to 1.9.x easy. Haven't had any crashes and it even feels faster.