I've been looking into getting myself some more RAM for my PC. So I looked online and watched eBay for a good price. So I made sure to look out for the exact same as what I have DDR2, PC2-6400, latency timings of 4-4-4-12, speed 800MHz, no ECC and unbuffered. I found an awesome deal with MicroXComputers on eBay, an american company selling 2GB DIMM's for $14 with free postage and packaging. I was well happy. So I bought it, got it all in one piece, threw it in my PC and wondered why it didn't like being in my PC with my two other Crucial 2 x 1GB PC2-6400 DIMM's. Tried it on its own, it worked grand, but in any other mix with the other two DIMM's, it simply wouldn't boot the PC, or it would take an age to get through the POST. I thought to myself, maybe I should have it as a pair instead. But checked around and it shouldn't be a problem.
Anyways, I went back to the listing and noticed this High Density listing it had for that specific DIMM I bought. I spent some time trying to figure out the difference between High Density and Low Density. I found out that Low Density are the better quality DIMM's and the High Density are the cheaper versions.
So you're wondering what are the differences. The difference is to do with how the memory on the DIMM is laid out. Each chip on the RAM DIMM represents a certain amount of memory like 16MB - 256MB. Low Density refers to the density of each of these chips and how they make up the size of the DIMM (1GB = 16 chips of 64Mx8). High Density would have a higher density chip (1GB = 16 chips of 128Mx4). So this must be the reason I can't mix them. I'm assuming the BIOS likes one or the other but not a mix. So I've gone and bought a second one of these high density DIMM's to test around with and get more information out of it.
The easiest way to tell if your DIMM is high density is if there's no name or manufacturer on the DIMM itself. The chips themselves will of course have a manufacturer like Samsung or something. There is no way of telling the difference between the number of these chips on the board as it really all depends on the density of each of the chips and the size of the overall memory on the DIMM itself.
So when looking to buy RAM, make note of the following
1) Speed, ie 800MHz
2) Data Rate, ie 6400
3) RAM type, ie DDR, DDR2, DDR3, RAMBUS (ignoring the older archaic stuff)
4) Density, ie Low or High
5) Latency, ie 4-4-4-12 or for DDR3 7-7-7-20 (otherwise known as the timings)
6) Is it ECC, Registered, fully buffered (slower for servers) or unbuffered.
7) What does your motherboard support?
Here's a quick glossary to the acronyms used above
RAM - Random Access Memory
DIMM - Dual Inline Memory Module
DDR2 - Double Data Rate version 2 (which in its long form is DDR2-SDRAM)
SDRAM - Synchronous Dynamic Random Access Memory
POST - Power On Self Test
BIOS - Basic Input\Output System
MB - MegaByte (not to be confused with Mb which is MegaBit!)
ECC - Error Correction Code
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