iMac 20" SSD upgrade options...?

I have an early 2008 iMac 20-inch model, and I'm running out of drive space on my 320Gb primary drive. For various reasons, I don't want to move things to an external drive. I am looking into options for upgrading the drive, and it occurred to me that if I can make an image of it with SuperDuper on my NAS, then I could install an SSD as my primary drive, move the original drive to be a secondary one, and restore the image to the SSD.

I'm having some difficulty finding online resources to help me with this. They suggest ways of installing an SSD but it appears that they are suggesting using it as a secondary drive, which makes no sense to me. If I install an SSD then I want it as my system drive, please!

So... can anyone suggest the best way to do this? I know it's fairly tricky to open the machine up and make changes, but I'm happy to risk that and I'm fairly confident in my abilities. My problem lies in the planning area: what should I be aiming for as a target system?

Are you even certain that your iMac 20" model can accommodate two drives? Best to go to iFixit: The free repair manual and look up the instructions for taking your iMac apart and see if they have instructions for installing a second drive.

You could replaced the iMac's internal HD with an SSD, largest 500GB. It would need the addition of spacers to convert from 2.5" to 3.5". If you then wanted to use your old HD it would need to be mounted in an external USB2 or FireWire case, FireWire recommended for access speed. Personally I wouldn't reuse an old 320GB hard drive from the iMac, they weren't very reliable. As you are running out of storage space why not simply replace the 320GB drive with a 1 or 2TB conventional drive.

Many thanks to both of you for the advice. I will check out the iFixit manual, and I'm also going to give some thought to Steve's idea about replacing with a 1Tb drive. I happen to have four of these just lying about waiting to be used, so it might make sense - but I want to check carefully whether they are compatible because I believe there's some issue with temperature sensing...?

On the other hand, some people are saying that it doesn't matter much as long as it runs cool enough. Maybe the WD Caviar Black would work OK - that's the model of the four drives I have. Any comments on suitability? They have all only been used for a few months, in my NAS RAID.