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This is going to be an unusual review. The product is Corsair CM2X4096-6400C5DHX memory which is a pair of 2GB DDR2-6400 modules that has a total capacity of 4GB. The DHX suffix denotes that the modules use Corsair's funky Dual-path Heat Exchange technology to keep the memory cool. You can buy CMX24096-6400C5 without DHX for £72 but you only pay a trivial £4 more for DHX so you may as well go the whole hog and splash out £76 for the full Monty version.
But that's putting the cart before the horse.
There are two strange aspects to this review. The first is that the rated speed of the Corsair is only DDR2-6400 or 6.4GB/second with a true clock speed of 400MHz. That's an effective speed of 800MHz which is perfectly adequate for any processor on the market however it's not the sort of thing that we usually review as memory of this speed is a commodity product that has little headroom for overclocking.
The second peculiarity is that this Corsair kit is 4GB (2x2GB) in size when most sets of memory are 2x1GB. There are a number of hardware and software issues that have lead to 2x1GB becoming the popular standard but the real kicker is Windows which uses a 32-bit addressing system that limits it to 4GB of memory (well, 4,294,967,295 bytes to be precise).
It's well worth reading up on this subject as the 4GB limit has historically been so enormous that it didn't affect PC users. In the days of Windows 98 we were limited to 512MB, and most of us were happy with 1GB to run Windows XP and find that 2GB is quite luxurious. However, Vista is a different beast and needs 1GB of RAM to run while 2GB is a sensible amount if you want to run games, edit video, or do a lot of multitasking. So many new PCs and laptops are already running at the ceiling of the amount of RAM that you can sensibly install.
Hopefully, you'll have spotted the flaw in my line of thinking which is that anyone who runs 32-bit Vista with 2GB of RAM has the option of upgrading to 4GB but, while that is true, the benefits of doing so are limited.
Whether you install 4GB in two 2GB modules or four 1GB modules it is most unlikely that 32-bit Windows Vista will show 4GB of RAM is present and correct. I shan't waste your time by recycling reams of technical papers from the net but there's an especially pithy summary here.
Here are the first couple of paragraphs:
I just bought a system with 4GB of physical RAM in it. The BIOS posts 4GB, but Windows tells me that I have anywhere from 2.75 - 3.5GB of RAM. Where is the rest of my RAM?
If you are running 32-bit Windows, you must live with it. You will not ever see all 4GB of RAM you've paid for.
If you are running 64-bit Windows, you may have to live with it. Depending on your motherboard's chipset, your system may support memory remapping. If so, you will be able to use all 4GB of RAM. ‘