What kind of RAM should I use for my iMac?
Getting a performance boost depends on what you actually do with the thing.  Run Activity-Monitor (Applications>Utilities) to see how much of your RAM is used.  Most Mac stuff runs well on 4GB, but if you run memory intense stuff (Parallels-Windows, Video Editing, editing large pictures with photoshop) you should see an improvement.  But even if you don't need the RAM, you should not see a performance penalty (i.e. it may or may not run better, but won't run worse).

Unless your computer is already slowing down to a crawl with the current configuration (when doing memory intense work), adding 2x 2GB should be enough (giving you a total of 8GB).

So, given your configuration, I'd add 2x 2GB to the free slots and keep the memory that is already there in the other slot.