Basically a SSHD is nothing more than a hard drive with a very deep cache (SSD). The drives hardware manages what should be held in this deep cache depending on how often it is called. So as an example your OS files and maybe some app files. If you constantly reload a given file it too maybe cached.
SSHD's are a good half step as larger SSD's are still quite expensive! A 1TB SSD drive is still over $400 U.S.
Some systems can't support two internal drives (i.e. HD & SSD) so a SSHD is a good option. If your system can support two SATA drives then getting a small sized SSD in conjunction with a standard HD could offer still better performance either as a fused drive (Apple Fusion Drive) or discreetly.
The other option here is a dual drive like Western Digital's Black2 drive. This is really two drives (HD & SSD) in one device. So far they only offer a 2.5" drive and is not very large and is a SATA III (6.0 Gb/s).
Here is the IFIXIT guide to replace your current HD: iMac Intel 27" EMC 2309 and 2374 Hard Drive Replacement.
If you are thinking of giving up your optical drive for a second drive this is the IFIXIT guide to follow: Installing iMac Intel 27" EMC 2309 and 2374 Dual Hard Drive and here is the IFIXIT drive carrier: 2.7 mm SATA Optical Bay SATA Hard Drive Enclosure.
Then you could go with a straight SSD in it and a standard HD to replace your current HD. I don't recommend going with a SSHD if you plan on using a SSD with it as most people want to create a Fusion drive set (combining the SSD with the HD as one virtual drive letting the OS manage the file placement). If you do plan on keeping the drives independent then there is no issues.
Here is the Apple P/N's for the thermal sensor cables if you go with a different HD or install a SSHD.
- 922-9223 - Hitachi
- 922-9224 - Seagate
- 922-9225 - Western Digital