If you are really good, you can fuse the SSD and the HDD to form a Fusion Drive. There are several articles online, one particularly good one by MacWorld. This will combine the SSD and HDD into one logical volume. OS X is smart enough to put the OS and your most frequently used files on the SSD, giving you the speed, and putting the large files on the HDD. This gives you speed without sacrificing size.
"If you have USB 3 and/or Thunderbolt you can use an external SSD. I just did that using a Sandisk SSD and Carbon Copy Cloner. Very straightforward and works a treat in terms of speed improvement.
Not worth it with USB 2.0 however, which is too slow."
Already SSD was replaced for 21 inch iMac , but without HDD sensor, am using Fan control apps (TG Pro ) to control the speed of fan, i don't know it will create any issues for iMac
