Memory

Most mainboards support DDR2 memory, but Core i7 boards require DDR3. Get the right type of memory for your board.

Memory has four parameters:

• A clock speed that controls the burst transfer rate. Memory transfers data twice per clock tick (that's what Double Data Rate means) so a clock speed of 400 MHz corresponds to DDR2 800 memory. Memory is also described by a standards notation based on megabytes per second burst rate, or 8 times the clock. DDR2 800 is also called "PC2 6400" memory. Old boards and servers may support rates slower than 800, while new boards may support faster rates. The clock speed doesn't really have much impact on system or application performance, and 800 is the pricing sweet spot.
• A latency indicating the number of memory clock ticks that the CPU must wait before the memory can respond with data at a new address. The most important latency number, often the only number quoted, is CAS latency and describes the delay when the new address is "close to" the old address (typically in the same 2048 bytes of memory, but this varies from chip to chip). CAS latency 5 is currently the pricing sweet spot for DDR2 800 memory, although you can find some CAS 4 memory. As the clock speed goes up, so does the CAS latency. PC3 8500 memory, for example, typically has CAS 7. If you do the math, you find that the faster clock cancels out the additional clock ticks and that all memory at all speeds has roughly the same latency delay (measured in amount of time rather than number of clock ticks). So you can get memory that runs at a faster clock rate, but it won't really be much faster.
• There is a standard voltage for memory. DDR 2 runs at 1.8 volts and DDR 3 runs at 1.5 volts. They don't always make it clear, but memory rated for higher clock speeds or lower CAS rates generally works only when given a higher than standard voltage. High speed DDR2 memory can require 2.0 or even 2.1 volts to run at the highest speed. The mainboard typically does not know this and starts out running the memory at 1.8 volts, DDR2 800 speed, and CAS 5 no matter what speed the chip claims to support. You have to change the memory parameters manually in the power up BIOS menus. If you don't do this, you will have spent more money to only end up with standard memory timings.
• Server memory supports ECC error detection and correction. It may seem like science fiction, but all the time high speed particles rain down from space. On average a computer chip will bit hit by one of these particles at least once a year. If it is a memory chip, it can change a 0 to a 1 or a 1 to a 0 and now your memory has bad data. If you are using your computer to play video games or watch a Blu-Ray high definition movie, there will be a short glitch in a corner of the screen that goes away in seconds. In a corporate server, however, such an error can screw up a payroll. So servers always have memory that can detect an automatically correct the single bit error caused by random chance, while desktop users typically don't do anything important enough to pay twice as much for memory. Server memory can also be "registered" or "fully buffered", which adds extra hardware and cost to allow more than two DIMMs to be inserted on a memory bus. Database servers can come with 32 or 64G of memory, but the extra memory at the higher price will cost you.

Memory has become insanely inexpensive. In late 2008 you can easily get 4 Gigabytes of memory for $50. With rebates, this can go down to $30. At this price it makes no sense to ask how much memory you need. You need 4G.  

Desktop memory comes in up to DIMM modules often called "sticks". The current generation of CPU chips and mainboards support two channels of memory with two sticks of memory per channel.

The lowest priced memory is a 2G stick. You can buy a pair of these for a 4G system. However, if you run a 32 bit version of your operating system you will not see all this memory. The hardware functions (video, disk controllers, LAN) use a half gigabyte or more of addresses. So don't think there is a mistake if your 4G of memory is reported as 3.2G by the OS. You can put 4 sticks of 2G memory to go to 8G, or 4 sticks of 4G memory to go to 16G on some mainboards (check your specs). However, this additional memory will only be visible if you run a 64 bit OS.

A laptop typically supports two SODIMM memory sticks (shorter and taller than a desktop DIMM). Many laptops support a maximum of 2 gigabytes of memory.