Now take the next step and create a Fusion Drive array with your internal SSD and external FW800. I have a 2008 iMac that I put an SSD into last year. I keep everything on it except my Users folder, which resides on the FW800 drive. A couple of weeks ago I decided to "merge" those 2 drives together to see how well they perform as a FusionDrive. It works great - better user experience than keeping them separate. OSX moves stuff around to optimize the performance of the machine. I now have a 2.24 GB Fusion Drive that acts basically like a huge SSD.
If you do this, just make sure you have backups of everything!
If you do this, just make sure you have backups of everything!