Questions about dual channel memory and the relationship between Ram and the FSB

I posted this in the Dual Channel thread, but I am actually trying to acquire information about other aspects of ram too, so I though I'd start a new thread.

I googled dual channel ram and found a wikipedia article which had a link to this article by kingston http://www.kingston.com/newtech/MKF_...whitepaper.pdf. It was pretty informative. It says "For the best dual-channel memory performance on motherboards with the Intel dualchannel DDR chipsets, you must use identically paired memory modules in DIMM sockets 0 of channel A and B. Identically paired memory modules must also be used when populating DIMM sockets 1 of channel A and B. One can, for example, plug in matching 256MB DIMMs in both DIMM 0 slots, and plug in matching 512MB DIMMs in both DIMM 1 slots."

Making the Dimm 0 slots have matching chips and making the Dimm1 slots have matching chips makes each Channel (A and B) as a whole have the same amount of memory. So since a notebook has two channels for it to be dual channel, the chip that goes into the single dimm slot of each channel must match the other. After reading this article which is probably from when the technology was new, I’m under the impression that there is not such thing as two dual channels. Each allocation of two dimms is a single channel, and the two allocations together make dual channel. This is what I gathered from reading the information in this article. Is this correct or am I mistaken?

according to the article:
A single memory channel can transfer 64 bits of memory to the CPU at one time while dual channel memory doubles that rate (128 bits). If the memory speed is lower than the front side bus speed, then the memory is a bottle neck in the system. According to the article a Pentium 4 with an 800Mhz front side bus speed has a peak data bandwith of 6.4GB/s (800MHz x 8 Bytes). Running DDR2 at 667Mhz with dual channel has a peak data transfer rate of 10.672 Gb/s (the formula is Memory Speed x Number of bytes transferred per channel x Number of Channels). That is way more than sufficient. Running DDR2 at 667Mhz with single channel has a peak data transfer rate of 5.336Gb/s. That might slow things down a little. However, if you were to run DDR2 at 800Mhz with single channel you would get a peak data transfer rate of 6.4Gb/s which matches the speed of the front side bus. So theoretically, if you were to run a 1Gb 800Mhz chip and a 2Gb 800 Mhz chip you would be able to run single channel memory with no decrease in performance.

1. My question is am I right? And if so does anybody know if my Inspron 1520's motherboard supports DDR2 800Mhz? I think I read somewhere that Santa Rosa only supports up to DDR2 667Mhz

2. If I will not be using all of the memory, would I be better off getting 2Gb instead of 3Gb? Is there any advantage to having more memory if your system does not currently max out your existing memory. Right out of the box my 1520 with 1 Gb running vista uses a little over 600Mb of ram when I'm not running anything. Does vista take as much Ram as it wants or does it only use a certain amount of the memory to save memory for other programs. So, if I were to get 2 Gb, would vista start using more memory? Would vista start using say 1Gb to run faster since there is still 1Gb left over for programs?

If having more memory will allow everything to use more memory and run faster, would that outweigh the slower bandwith and make it worthwhile to get the 3Gb?

I have no problem spending $50 extra for the 3Gb, even if it will only have a slight increase in performance. The thing is that I will probably be using less than 2Gb 90% of the time anyway, so if the lower bandwith would slow the computer down during this time, I dont want it.

Another thing that someone told me in the Dual Channel thread is that with an 800Mhz FSB and dual channel you only need 400Mhz Memory to match the bandwith of the FSB. And you can buy 400Mhz memory with tighter timing. I dont know exactly what all of the timing numbers mean, but I think lower timing means it will run faster. If you were to buy 667Mhz ram, would it automatically downclock and lower the timing? If this is the way it works, why would dell sell the computer with 667Mhz memory if it will not provide any performance gain over 400Mhz memory which is available with lower timings?

 

1) The article is right, however, you will notice very little performance difference with Dual vs Single channel on laptops. Most likely it doesn't support 800MHz. The only Santa Rosa laptops supporting that is a Zepto laptop (and we are still trying to figure out how ). Anyways, the differnce between 667MHz and 800Mhz is very little anyways.

2)The more memory you have, the more Vista gets. But it'll free memory when needed (though it'll still give lesser free memory compared to XP). And the 3GB memory will do better than 2GB (not on speed, but by having more memory, you will run memory intensive applications better).

It's not like that at all.A DDR2-800 memory always runs at 400MHz (The DDR stands for Dual-Data-Rate , meaning that it transfers two signals at each clock, so the effective rate is twice the actual memory clock)

 

I think a little confusion going on here, mixing apple's and orange's. Aren't notebooks still only at 667Mhz (Intel) so the 800Mhz FSB does not relate to the memory controller. When memory speed is talked about what do you mean? 667Mhz is a 333 frequency but because of Double Data Rate it is called 667Mhz, so how you get your # is important. Dual Channel is not cumulative, you do not add up the speeds. Also if current DDR2, naming convention is different 400Mhz is a DDR convention and compare to 200Mhz if you want to start doubling. Not a complete answer but I am not an expert. Maybe add more later help it helps or gets you to ask again to clarify. I think the question is confusing because does not make sense your FSB is not important for your specific question.

Edit: posted b/4 I saw mujtaba, I guess he typs faster.

 

refer to this thread

http://forum.notebookreview.com/showthread.php?t=157819

or the other one about dual channel ram I think i have asnwered everything there is to know about ram already