when DDR SDRAM chips hit 512 megabit densities, is when you'll see 512 meg DDR SDRAM sticks at most frequencies.
I think only recently 256 megabit chips have become more common.. that is, 32 megs per chip. I know someone on this forum had a 128 meg stick with only 4 chips on it.
I have a 256 meg PC133 stick of SDRAM, it has I think 16 chips, 8 on each side. so that means that each chip has 16 megs on it.. this stuff was ultra cheap though (7.5 ns, runs CAS 2 fine with 133mhz, probably wouldn't do so well with higher frequencies).