Sorry to say you're between a rock and a hard place here ;-(
The optical drive your system has uses a PATA connection to the logic board so it's painfully slow! While a second HD would work here (2.5" 7.5mm high) as additional storage it would not be very effective as the boot drive.
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So your options should be to replace both drives! Putting in a SSD (2.5" with a adapter frame) where your current HD (3.5") is, then put in a new HD (2.5") into the optical drive HD carrier replacing your optical drive. Frankly, that's very expensive and you won't get that much bang of improvement as the SATA port the primary drive port offers is only SATA
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So your choices here are replace the HD for a SSD, or go with a SSHD hybrid drive which I think is the better move. Here's what I recommend: [http://www.seagate.com/internal-hard-drives/solid-state-hybrid/desktop-solid-state-hybrid-drive|Seagate Destop SSHD]. I would recommend sticking with the 1 TB drive as I doubt you need that much storage. Here's the IFIXIT guide to replace your drive: [https://www.ifixit.com/Guide/iMac+Intel+20-Inch+EMC+2133+and+2210+Hard+Drive+Replacement/1008|iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement].
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If you deside to go with the SSD instead, you'll need a adapter frame to hold the 2.5" SSD in the HD's 3.5" bay. Lastly remember to trasfer the thermal sensor over as the system needs it.
Sorry to say you're between a rock and a hard place here ;-(
The optical drive your system has uses a PATA connection to the logic board so it's painfully slow! While a second HD would work here (2.5" 7.5mm high) as additional storage it would not be very effective as the boot drive.
So your options should be to replace both drives! Putting in a SSD (2.5" with a adapter frame) where your current HD (3.5") is, then put in a new HD (2.5") into the optical drive HD carrier replacing your optical drive. Frankly, that's very expensive and you won't get that much bang of improvement as the SATA port the primary drive port offers is only SATA