Thanks for the advice. I really like the fact that people inhere don't flame people like me, who ask intrinsically stupid questions
I managed to install the RAM. My experiences was:
a) The RAM access door was almost impossible to get off, really. I used a screwdriver to un-screw the screw, but because the door was fit in so tightly, it didn't come off and since I didn't know the construction, I had no idea why. I managed to use a small metal pin of some kind to actually press the door out. When I had it off, I realised that the metal was actually bend, because the screw had been pushing outwards, but the door was fit in so tightly. Hard to explain - anyway, I was able to bend it back to normal again. I wouldn't say that I could do this exactly as descriped in the manual. It is not a completely idiot proof procedure.
b) There was no click while inserting the RAM. I removed to old module too, just to insert it again and check it out. At some point, I felt it go all the way in (this sounds wrong..), but there was no click. Regarding the new module, I was not able to feel the "all the way in" sensation nor hear a click. I just had to push until I could feel, that it didn't move any further in, and it looked like it was in as deep as the other module. I thought that i'd better take then both out and try again, just to hear that clicking sound, but right now the modules won't really come out again. Instead of the system used, I would prefer something like what was in the old desktop computers - you know, where it was easy to put in new RAM and you were absolutely sure, if it was fit in place or not.
Anyway - here I go with my new iMac (from yesterday) and 3 gb of ram.