Quote from joe:
That should work just fine. You would just select the SSD in Startup Disk as soon as you get the drive in there with OS X installed.
There should be a 3.5" drive with both an SSD and a high capacity HDD on it (dual SATA). The SSD wouldn't need to be that big, say 32 or 64GB , since the most active files and swap space would reside there. I guess it would only apply to the iMac though.
Given how much the SSD in my MBP has sped things up, I almost feel installing an SSD in a new iMac is a necessity.