that you will most definitely benefit from extra memory, providing you DO get the correct modules.
If you get say, 256MB or larger, it will probably be safer to remove what you have in slot 1 and put your biggest memory stick in this slot. Then put your smaller one in the second slot.
You will have problems if the two memory sticks are not compatible.
To remove, or install memory modules, you will have to use the little tabs at each end of the slot. Pressing these down will eject the memory stick already in there. When putting new memory in, press it down firmly until the little tabs click into the slots on the module. (You might have to manually click these in place, but make sure they are aligned with the slots on the memory module.)