For what it worth, I just got a 24" 2.4 GHz iMac with 1 GB of ram and I purchased 2 x 2GB from OWC. I was surprised at how easy the memory upgrade was. However, I did make sure the computer was running before loading the RAM, that was 2 weeks ago with Tiger. Now I'm running Leopard without a hitch.
This is not meant to disrespect the OP, but I was in nearly the same situation but I was surprised at how smoothly it went. Now my Mac Mini is a bit more involved. Still, I feel your pain, for what that is worth.😱