Unpack ruby-enterprise-1.8.6-20090610 somewhere and install it with:
$ ./installer --auto=~/ree186 --configure-arg 'ARCH_FLAG="-arch i386"'
Double-check your installed ruby with:
$ file ./ree186/bin/ruby
./ree186/bin/ruby: Mach-O executable i386
A problem I had while double-checking that this worked is that RVM had changed my gem home to something the installer hadn't expected. Just something to look for if you've been playing around with various methods of installing multiple versions of ruby. Outside special cases like this one, it works quite well.
As for ImageMagick, if you use macports, make sure you install it using the 'universal' variant:
$ sudo port install ImageMagick +universal
Actually, it will be significantly less pain in general if you just deal with the increased compile times and install everything universal in macports. Create a file at /opt/local/etc/macports/variants.conf containing '+universal' and any other variant flag defaults you want to set up. Open /opt/local/etc/macports/macports.conf and make sure 'universal_archs' is set to 'x86_64 i386'. After setting this up, make sure you install everything that you need via macports to avoid additional headaches.
There are also plenty of reasons to attempt an upgrade of rails to 2.3.4 if the level of complexity and test coverage are sane.