I bought an iMac in 2007, a 24inches one. With 4 Gb Ram and a 320 Gb disk.
Recently, I really started to get annoyed by how slow it is. I have a 2010 11" Mac Book Air that's faster for everything thanks to its Ssd. And gong from one to the other was really annoying.
So I started to look at options to speed things up, notably on owc. But they were not nice to me, and told me the truth: my iMac is too old to benefit from an upgrade package.
I just had the hard disk replaced, and it's already much better. But I'm in for an upgrade soon, at least in 2012 (finances permitting).