He was having issues with his mac being very slow and was running an older OS.
I replaced the original 3.5 hdd with a 2.5 ssd and used a hdd adapter. I also installed an optical drive to hdd adapter sold by ifixit and placed a 500gb hdd here. The reason I swapped the two is on this iMac the optical drive is a pata connection (very slow) and the original drive is a sata connection ( very fast). Last I installed 4gb of ram. This machine is now running mavericks OS and is extremely fast. Start to finish was about 3 hours including os download reinstall and data transfer.
The best advice to you is do all of your research first. Make sure you read everything before you buy. They will tell you everything on the ifixit.com but you call them if you have questions (very cool people). Also pay attention to each step on teardown, don't get ahead of yourself and you will be fine. Macs are so easy to work on especially when they give you step by step pictures. Ifixit has most everything you need to get the job done but there might be something they don't offer (3.5 to 2.5 hdd adapter).
12.7 mm PATA Optical Bay SATA Hard Drive Enclosure
PC2-6400 2 GB RAM Chip
Heavy-Duty Suction Cups (Pair)