In my system the only way to get an SSD drive (OCZ Agility 3 120G) to work was to install it to the first SATA interface and the original HD to the second SATA interface.

I tried first as said in this guide, but the whole system got very unstable, the SSD was basicly unusable with lots of read errors, kernel panics etc...

After switch everything went smoothly. I cloned the original system to the new SSD with Carbon Copy Cloner, selected the SSD to be the startup disk and rebooted, Now the SSD is primary drive (with huge difference in speed, some 340/300 read/write) and the original 1T HD is secondary drive hosting my iTunes, iMovie and iPhoto archives.