Despite my wife's early 2008 model iMac getting the 6gb ram upgrade, she was still complaining about it being too slow in photoshop 5. (Specifically the healing brush)
It was fairly easy once the glass came off... the photo tutorial showed me pretty much everything I needed.
If your putting in a 2.5" SSD drive, you'll need a 2.5 to 3.5 adapter. Also when putting the aluminum case back onto the i-mac there is a seal that goes around where the memory access panel is... make sure to tuck the seal back in or else the memory access panel won't fit back into place.