There is a PC-133 (144 pin) SO-DIMM that is user accessible via the 4 (unremovable, they just loosen up) phillips screws on the bottom plate. Just tip that bad boy on its side (unless something is broken, it's OK to grab it solely by the "arm", or better yet to have one hand on the "arm", and one on the base of it) and place the monitor face down on something soft and even (maybe the middle of a bed, or better yet some cloth/foam laid down on a table or desk). This will give you access to the bottom.
So you can start by placing a 512 chip in there, as once the plate is taken off the RAM is right there. I believe there are even little instructions on the plate, either on the outside of it or on the inside (I don't remember).
Not really recommended for the other stick....
The un-user-accessible stick is the bigger "desktop version" of PC-133, (168-pin) SDRAM. With the appropriate hex (I think that's what it's called) screw driver via another 4 screws. You'll see them with the bottom plate removed.
That entire bottom section of the base (you can see the seam or edge about 1-inch from the very bottom going along the entire base, just above all the ports). That bottom section of the base is where the other RAM stick goes, which is easy enough to get open, but the crazy part is that it also has the logic board and all the cables going from it to the upper section. So in that sense you can't completely separate the bottom section from the base (you can, but you don't need or want to). However, as per the reason mentioned below, it can be kind of tough to separate or un-stick that bottom section even with the screws removed. It takes a bit of pressure depending on how lucky you're feeling.
While that's not too bad for most techy people, the downside is that the heatsink from the processor also connects that bottom section to the upper section of the base. It IS recommended that because you severe that connection, you have some thermal grease and a putty knife to remove the previous gunk, and reapply a thin, fresh layer of your own.
While I have done it and forgotten to not reapply that thermal grease, I would recommend it otherwise.
While searching google, it should be easy to find a "flat panel" "imac" "take apart" guide to at least get you from point A to point B.
Although, I have probably scared you off completely from installing that second stick