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Depends on how much is loaded. I have a 2007 iMac at 6GB and that 2GB was enough to get it to go from a crawl to a walk on the current OS X. It should be cheap enough by now to extend the usable life. You have a 2008 so it should behave better than mine.
 
As long as you've already got an SSD in there, it probably won't be drastically different. And new 4GB sticks of DDR2 are quite expensive, best to eBay it I suppose.

I have a 2007 base model 20" iMac with an SSD and 4GB Ram and it runs just fine. I also had an Early 2008 MacBook that I did put 6GB in but the difference between the two running the same apps was minimal other than the terrible Intel GMA on the MacBook.

You might benefit more if you run virtual machines or edit large RAW photos or panoramas in Photoshop. But for day to day use on these, it wouldn't be worth it to me. And I run a lot off that '07. Logic Pro X, lots of Adobe apps (PS, ID, IL, LR, etc), Office 2016, Serato DJ, Aperture, some FCPX, etc. Runs fine in El Capitan. Of course, everything was better in Snow Leopard or even Mavericks, but software support is somewhat important. Mainly an up to date browser, so I stick with the latest.
 
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