PC-133 memory Mbit rating. How do you work it out?

I'm trying to add memory to my computer which takes PC-133 SDRAM by
buying some second-hand. According to my motherboard manual it will
only recognise a DIMM size of 256 MB if it's using 128-Mbit device
technology.
So my questions are, is there any easy way to tell if memory is
128-Mbit, and what's the difference between 128-Mbit & 64-Mbit
memory?

After doing a bit more research, I'm getting on the right track to
finding an answer. I suppose I just need someone to explain the
numbers to me.

For instance I found some memory advertised like this:
PC 133 (32x64) 256 MB
256 Megabyte 168-pin 3.3 Volt Non-Parity 133MHz SDRAM

And then some more expensive memory from the same company described as
such:
PC 133 (32x64) 256 MB 16 chip (16x8)
256 Megabyte 168-pin 3.3 Volt Non-Parity 16 Chip (16x8) 133MHz SDRAM.

Looking at my motherboard manual, it lists different memory
configurations, but I'd really like to know what the numbers mean.

Thanks,
Dash

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