We have an early 2009 model 24" iMac that had recently started to perform very slowly. It only had 4gb of RAM and the hard drive was getting a little noisy so I was concerned that it might go out soon. I decided to try both maxing out the RAM at 8gb and swapping out the HDD for a solid state drive.
The repair went swimmingly with the guide I found on iFixit for replacing the hard drive in a 20" early 2009 model iMac. Found here: iMac Intel 20" EMC 2266 Festplatte austauschen
The repair took about an hour, and probably would have been shorter if I hadn't had to reposition the solid state drive in it's special chassis a few times to make the cables reach.
Wear gloves and try to handle the front "glass" and display as little as possible. This will make cleanup prior to re-installation much easier. The cabling is a tad more complicated on the 24" model, but still very manageable, just remember to hook everything back up as you re-assemble!
Universal Drive Adapter
TR8 Torx Security Screwdriver
TR6 Torx Security Screwdriver