I agree, just go to <A HREF="http://www.crucial.com" target="_new">http://www.crucial.com</A> and use their module selector because some motherboards have incompatibilities (especially AMD baords). But just to answer your question...
(# of memory addresses)x(# of bits/address)
16x8 refers to the chip density, which is actually 16Mx8, 16 million memory addresses of 8 bits each or 16MB per chip. A 128MB RAM module should have 8 16x8 chips, and a module density of 16x64. Some 128MB RAM modules have 16 8x8 chips (FYI, 8 16x8 chips are better than 16 8x8 chips).
Most likely the two numbers you are referring to are the chip density of one and the module density of the other. They probably have the same chip density, module density, and number of chips.
Hope I didn't confuse anyone too much