First and foremost: Add more RAM! Max it out. At this point the RAM for that model is pretty darn cheap and it is easily user installable.
I have a strong suspicion that the RAM upgrade will solve your speed concerns- I have the early 2009 iMac with a 2.66mHz Core 2 Duo and it really suits my needs just fine. An upgrade to 4GB made a HUGE difference on my Dad's iMac running Mavericks. (I upgraded mine to 8GB so long ago I don't recall what difference it made)
I have not made the leap to Yosemite or SSD yet. I just ordered an SSD (Cyber Monday pricing!) and will do a clean install of Yosemite when I get it, so the following is from my research and NOT personal experience.
From what I gather the SSD will give a big performance boost for launching and switching apps as well as apps like the Microsoft Office programs that frequently read from the hard drive. A hybrid SSHD will give only a fraction of the performance boost of an SSD.
I use a big external HD for Media storage, so I don't need a huge HD in the machine. If you need a lot of internal storage space, an alternative to SSHD is to replace your optical drive with an SSD and keep (or upgrade) the spinning HD. Which is best will depend on your needs and how the machine is used. (The optical drive can go into an external carrier in case you occasionally need it. I wouldn't go this route if I used the optical drive frequently!)
Here is a BIG consideration if you decide to go to SSD: If your iMac is from the 2009 time period, a lot of SSDs use a controller chip that interacts poorly with the SATA controller in the iMac. I cancelled an order for a Kingston SSD and instead ordered a Crucial M500 after learning about this. The M500 supposedly does not have the offending chipset.
Don't forget the drive bay adapter since the drive in your iMac is 3.5" and the SSD is 2.5". There are a variety of Bay Adapters to choose from, but you will want to have one that puts the connector in the same physical location (iMac internal cables and connectors don't have slack in them for re-positioning.) Based on reviews, the Newer Technology Adaptadrive seems to work well with iMacs. For a 240GB SSD and adapter, I spent under $100 including delivery and sales tax! Local guys will install it for $100 but I may give it a go myself.
Other issues I am digging into with the SSD are the possible need to use a third party driver to enable "TRIM"; having to disable some Yosemite security to be able to run TRIM (you can read about this at a company called Cindori); and whether I have to worry about the heat sensor that is attached to my current HD. I will report back when I finish this project and let you know what I learned and did.