Hi -- I've got an Early 2008 iMac, also.
Hard to answer your question. Monitor your total memory usage with a menu-bar program like iStat Menus or even use Activity Monitor (change the dock icon with View-->Dock Icon-->Show Memory Usage).
If you use virtualization software like Parallels, VMWare Fusion, or VirtualBox to run guest OSes it's likely you could use the extra RAM. I put 6 GB in mine and that's the only time I've noticed that I really use the extra. (I'm running OS X 10.8.5 right now.) However, I'm not the type to leave iMovie, iPhoto, 103 browser tabs with flash going, etc... open all at once.
I partly maxed mine out because I hope to keep the machine for a few more years and thought that memory for an older machine might actually get more expensive as time went on. Don't know if it's the case, though.
Did you put the SSD in yourself? I put an SSD + 1 TB HDD in mine and made my own DIY Fusion Drive. So far it's been great!
Brian.