I just tried 64-bit opera on my Gentoo setup (it defaults to installing the 64-bit version) and it seems pretty snappy, and it works with the new 64-bit flash plugin too
Unfortunately though, icedtea (java built for 64-bit linux platforms) doesn't seem to work with it yet. I haven't used opera for a while now, but I'm impressed at how well it has come along.
I'll stick with firefox for now I guess, until icedtea works with it.