If you find that the Mac isn't fast enough, use standby more often instead of shutting down. Since everything will remain in memory, it will be quicker in operation than after a fresh reboot, and since you have 8GB of memory, that shouldn't be a problem.
The iMac only has one hard drive bay, so you can't have both an SSD and a regular hard drive. Your best bet, if you really do want it to be faster without costing you the earth and restricting you on storage space, is to install one of those hard drives that have a small amount of flash memory onboard. It's kinda like an SSD and HD hybrid, the best of both worlds.
If you do replace the drive, stick the one that came from the iMac in a USB caddy and use it for backup.