Determine Correct HDD Replacement

To start with you should check your systems firmware to make sure its up to date. Follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers

As to what drives you can use you'll need a drive which runs at SATA II (3.0 Gb/s).

Seagate no longer uses the Barracuda name so the drive maybe an older model. Here is the Spec sheet for the current line of Seagate HD's: Seagate Desktop HDD. You may want to think about going with the SSHD version of the drive. Here's the spec sheet for it: Seagate Desktop SSHD. They don't make a 3TB version so you'll need to jump up the the 4TB drive or go with a 2TB. The benefits of the deep SSD cache allows the system to boot up faster and often used apps.

For either you can see the spec sheet line: SATA Transfer Rates Supported (Gb/s) has 6.0/3.0/1.5 these drives have auto sense technology to match up with the systems SATA ports speed. So both are good choices. If you decide to use a different drive make sure it can do the same or can be set for SATA II (3.0 Gb/s).

Lastly, here is the IFIXIT guide: iMac Intel 24" EMC 2267 Hard Drive Replacement