Hello, i wanted to get performances on Final Cut Pro X with OS X Lion 64b. It's done now with your kit.
I have bought an iMac21 refurbished (-15%) from AppleStore, your iMac Intel 21.5" Mid 2011 Dual Hard Drive Kit from iFixit, and an Intel® 180Go SSD 2.5" 520 Cherryville with SATA3 from Paris. I've upgraded it in 3 hours. Every step reading the doc. At boot OS X Lion the system included the SSD. Plus a Carbon Copy Clone to mirror the OS on SSD, and so next reboot : finished. The iMac 21 boot immediatly without noise. Great success.
Take your time with ambiant light and no stress. It work