What are the common kinds of memory used in PCs today?
All user-upgradeable memory in PCs these days is a variant of dynamic random access memory (DRAM). The most common types are Synchronous DRAM (SDRAM), Double Data Rate (DDR) SDRAM, and Rambus' RDRAM. RAM has a maximum speed, like processors, and plugs into a set of signals in the computer called the front side bus. SDRAM produces data at the same speed as that bus and is named accordingly - PC100 is 100MHz, PC133 is 133 MHz and so on. DDR RAM produces two lots of data per Hz of bus speed, so is up to twice as fast for the same bus. RDRAM has a unique very high speed serial bus, and runs fastest of them all.
What do the other numbers in a memory type mean?
Chips are referred to by their speed, while modules are called by their bandwidth. So, a DDR chip that can work at 200 MHz is DDR200, while a DIMM made from DDR200 chips is called PC1600 DIMM - it can produce 8 bytes of data every clock. Most Pentium 4 systems use PC2100 DDR, which costs in the region of £100 for 512MB. You'll also find CAS figures mentioned: these are the number of bus cycles required to set up the chip for data disgorging. 2.5 is good, 2 is better.
What's the fastest memory I can get?
That depends on the chip set. Intel has said that its 845 chip set will support 333 MHz chips in the very near future, while VIA, SiS and Nvidia have chipsets that will support DDR400 - although this is not quite an approved standard, and should only be considered if you're keen on experimenting. It can approach, and in some cases exceed, the figures for Rambus' PC1066 memory, which is still the fastest established standard.
DDR-II is a new standard that gets four times as much data per clock as SDRAM. It will need new motherboard designs, new chipsets and new sockets, and will compete directly with RDRAM. There may be an intermediate version called DDR 1.5, which is old-style DDR chips on a DDR-II package and which will provide slower but cheaper options for new motherboards.
Can different memory types or speeds be mixed in the same system?
Almost always no - although you can sometimes mix low and high speed variants of otherwise identical memory, it'll all run at the lower speeds. Some PCs can use either SDRAM or DDR-RAM, but only one type at a time.
What do the various memory settings in my BIOS mean?
You'll probably find CAS latency, RAS to CAS Delay and RAS Precharge, which set up the number of bus clocks needed to let the named signals settle down before the memory can use them. Memory chips are also specified with these numbers, and by matching the BIOS settings to the memory specs you can get the most out of your system. You can also make things remarkably unstable, if you make the timings too tight.
What's ECC memory? Do I need it?
ECC - Error Correcting Code - memory not only detects single-bit errors but also corrects them, at a suitable cost and complexity premium. They're normally found in high reliability servers and other systems where an uncorrected error could have significant financial consequences - so if you have to ask, the answer's no.
SIMMS? DIMMS? SODIMMS? What?
Memory packages have a variety of acronyms that reflect their physical appearance - SIMM is Single Inline Memory Module, because it has one row of pins at the edge; DIMM is Dual Inline - it has two rows of pins -- and SODIMM is Small Outline DIMM, used in laptops. SIMMs tend to be installed in pairs and are the oldest form factor of the three, DIMMs are higher capacity than SIMMs and can be installed singly, while you'll rarely find more than two SODIMM sockets in any system.
Will more memory always speed up a PC?
In general, more memory reduces a system's dependence on temporary disk storage while software is running and thus speeds things up. However, under some variants of Windows adding more memory can have little effect or even slow things down -- Windows 9X will not run properly, if at all above 1GB - and if little use is made of the hard disk in use, then extra memory won't help.
Why are memory prices so variable, and will this change?
Although memory use by games consoles, set top boxes and other devices has to some extent offset the fall in demand from the PC market, DRAM makers are still oversupplying the market and costs are subsequently variable but low. For the foreseeable future, memory will remain a commodity on a buyer's market.