Here's the IFIXIT guide you'll need to follow to get to the HD: iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement.
Given the age of the system I would recommend going with a SSHD (hybrid) vs a SSD. Review this: Seagate Desktop SSHD. You'll gain the speed of the SSD but also have the storage space of a HD. A 256GB SSD is on the small side as the only drive in the system.
You do have a few issues when going with a SSD as the your systems SATA port is only SATA II (3.0 Gb/s) so what ever drive you put in it must run at this I/O speed. The Seagate drive as an example auto senses the SATA port of the system and sets its self to it. The second issue is the way SSD's work you'll need to install a special driver to enable TRIM services and you will likely need to upgrade the OS as well.
Make sure you put back the thermal sensor onto the new drive (HD or SSD). You'll encounter issues with the SMC services (Fans) if you fail to reattach it.
OK the RAM is the next issue here:
You may want to push your system all the way to 6 GB using one 4 GB module and one 2 GB module.
The last thing here is making sure you have updated your systems firmware before doing anything. Follow this Apple T/N: EFI and SMC firmware updates for Intel-based Macs.