Matched Memory Modules

[HMSDOC]

I am hoping that someone might help me out or at least point me in a direction
where I might be able to learn a bit more about memory. I am preparing to
build my first PC and am looking to build a relatively "high end" box. I am a
bit confused about memory and searching the internet has not helped clarify an
issue for me. The motherboard I am looking at (MSI 865PE NEO FIS2R) with an
800 MHz FSB can accept up to 4GB of DDRAM into 4 slots. My confusion arises
from the issue of 'dual channel'. I understand the concept (I think) that with
dual channel memory 2 modules are essentially accessed at the same time
leading to full use of the 800 MHz FSB. I realize this is probably an
oversimplified view but that is pretty much what I gather from what I have
read.

My confusion is to what the 'matched pair' memory being sold is? If I want the
system to have 2GB of RAM I can put in two 1 GB DDR 400 modules into the
appropriate slots to pair them. But what are the matched pair modules that are
offered by some companies? How are they better…what do they do better? Will
using unmatched pairs still result in using the dual channels? I also noticed
that the largest modules in matched pairs that I see come in two 512 MB modules
meaning that one would need to fill the 4 slots to achieve 2 GB but then this
would max out the amount of RAM the motherboard will accept without going up to
4 GB….I am sure I am misunderstanding something here and would appreciate any
help. Thanks.

Howard

 

[mdp]

When using memory in pairs, like older computers do using 2 SIMMs to get a
64-bit word, it's important that the timing characteristics of each chip or
stick are the same, i.e. rise times, fall times, latency, etc. When modules
or chips are said to be "matched", then the chips come from the same
"family", usually a lot, sometimes even the same wafer. Wafers in the same
lot tend to operate more like each other than compared to the same design
but processed in a different lot. Processing wafers takes a lot of steps
and controls and there is variation along the way. All added up, this
amounts to small but significant differences if the two chips (or sticks)
have to operate together in a tightly synchronized fashion.

 

[HMSDOC]

So what type of performance differences would one expect between:

a)2 unmatched 1GB DDRAM 400 modules in the dual channels OR

b)2 matched 512 MB DDRAM 400 modules in the dual channels OR

c)2 sets of matched 512 MB DDRAM 400 modules for a total of 2 GB using all 4
slots?

Also why don't the matched pairs come in 1GB sizes (two 1GB modules)?

Howard