A through D G3 iMacs used sticks of memory similar to that found in a laptop computer and were rated as PC-66 or PC-100 standard. You will have to pull out the logic board cage assembly in order to change out the RAM on these models. The 350 Mhz through 700 Mhz G3 iMac models all used desktop memory of PC-66, PC-100 and PC-133 standards. For these models, you only have to unlock a door underneath the case and pull it open to get to the RAM; it's that easy.

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Right, i only got the slot-in type to play with.

Concerning ram, you cannot make any mystake as they are quite different in size and shape, effectively the very first generation of Imacs came with so-dimm sticks (found on ibooks and powerbooks of the same generation) where slot-in imacs used the common Sd-ram found on dekstops and on pc's (even if not always a 100% compatible)

another try is to leave only 1 stick and try to boot (if several fitted)

you only have to unlock a door underneath the case and pull it open to get to the RAM

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Right again, you just need a coin to release the latch and have access to ram and airport slot.

Another tip, just above that door, you should have a perforated blanking plate, to gain access to a vga output (if available, as not all are supplied with)

try to connect a screen and fire the Imac, an upgrade to Mac Os X without loading the proper firmware tends to disable the inner screen.

A firmware upgrade (from a HD and 9.2 Os) solves the problem.