IMac :: Upgrading Processor Due To Slow Performance?

I've just installed an SSD in my new iMac 27", see here. I'm fast learning though (diglloydTools and other commentators) that once the application has loaded the SSD isn't involved very much from there on, so there are the same old performance problems. I'm finding in Firefox/Firebug where there is a large DOM (nothing exceptional) performance is still woeful.

So, baring in mind the Core i7 processor is under $200 and it would appear to have been possible to upgrade the iMac's processors in the past, does anybody have any advice for me? What parts will I need, have you done it and what was your experience, any common problems with doing this, are the parts themselves upgradeable (the quoted link only suggests they are) etc.