Just buy another 128MB of PC133 RAM, 99% garanteed to be compatible (PC133 will run slower if needed to, as I don't know the FSB of your system). To make sure though, get some Crucial. It won't matter what slot you put it in, or if you just use 1 2 or 3 slots. SDRAM mobo's are not too prone to compatibiliy problems as far as that goes.
To make it more helpfull, if you can, give us the chipset name to make sure that there are no known problems with it.