I did not have enough storage in my iMac and constantly being connected to external hard drives not only was restricting but degraded performance.
I installed a 2TB WD Green HD to replace the stock Seagate 1 TB and removed the optical drive in favor of a Sandisk 240 GB SSD on my Late 2009 27" iMac. I then booted into recovery mode, I did have the OS on a SD card in the card reader. In terminal I entered the correct commands to create a fusion drive and then recovered from Time Machine. After that I transferred over music, movies, photos, Final Cut Events, and anything else I wanted onto the internal fusion drive.
Be careful, the very first cable that is pulled out is unlike any I have seen, it just looks like a small ribbon. Also, I purchased an optical drive thermal sensor for the WD HD. I did not want my fan running at 100% all the time and the jumper connected thermal sensor is proprietary. I am using OS 10.8.2 (12C60) and fusion is fast fast fast. Fast boot, fast app launch. Working on movies and photos is faster too, however I can probably attribute much of that to files being on the HD as opposed to a firewire or USB connection.