It is almost comical, though that, just when memory prices were at their peak, a new generation of DIMM has become available boasting a solid 256 Mbytes in unbuffered form. The keyword here is unbuffered, since buffered a.k.a. Essentially, what it comes down to is that there are additional register chips on the individual DIMMs. The function of these register chips is to serve as interface between the chipset and the individual memory chips. To answer this question, a bit of background is necessary on memory architecture.
Memory is nothing but a high density array of capacitors that can be charged.
The usual package format is a memory chip. Every memory chip consists of addresses and bits. Thus if there are 16 mega-addresses x 4 bit, the chip will have a density of 64 Mbit. In order to get up to 128 Mbyte DIMMs, consequently the number of chips needs to be doubled, usually resulting in the configuration of double-sided DIMMs with eight chips on each side. In this case the DIMM is usually configured as 2 bank module.