Couple of tips. First, I find I get less frustrated working on a G4 iMac by holding it between my knees upside down.
Second, getting the bottom back on is a bear, because of the multitude of cables, some of which are barely long enough and require dexterity and vast quantities of patience while reseating the bottom.
Third, there is a spot where you need to scrape off the old thermal paste (use something non-scratching, like a spudger) and replace it with fresh. So do some googling to find more thorough instructions.
I agree - I've done this upgrade before on my 1.25GHz iMac G4 - it's not that hard to do, although you DO need to be careful of the cables joining top/bottom of the case.
some additional photos more clearly showing the cables can be seen here
Main thing to note is that the factory installed so-called "non user-upgradeable" memory is a full-size 184pin SDRAM DIMM, whereas the user-replaceable one inside the bottom hatch is a 200pin SO-DIMM