Well, I did the get the Crucial M4 128 GB SSD installed and it's working beautifully. Taking the iMac apart was tedious but very easy. Here's my tips: 1. buy the torque screw set from ifixit.com or somewhere online (but why not ifixit), you can't find a screw set that small anywhere like home depot or radio shack 2. the crucial 3.5 adapter kit does not fit from crucial we had to cut a large piece of foam and place it behind the kit to secure one end from moving up and down (not may favorite solution, but it's very secure) 3. a third party SSD will NOT have trim enabled with Lion, be sure to find one that has a fairly decent garbage collection feature installed on the disk itself. not sure if this is going to cause a problem or not 4. also, the thermal sensor is on the SSD and the HD fan is not spinning up crazy. It's nice and quiet SSD is great. Applications open in about a second or two. It's CRAZY!!! Never going back.
The SSD I'm getting comes with a 2.5 to 3.5 adapter. So, I'm good there. But my iMac is before 2008/2009 so the thermal sensor is supposed to work so that the fan doesn't fun at full speeds. I have you tried installing an SSD on my particular model iMac Intel 20 Inch 2133?