Memory (RAM)

There are three standard types of RAM used by home PCs:

SDRAM (PC66)
RAM with a 168 pin connection to the motherboard. Can only run at up to 66Mhz motherboard speed (aka frontside bus) and is the same price as PC100 SDRAM - in other words, not worth buying.
SDRAM (PC100)
RAM with a 168 pin connection to the motherboard. Can run up to 100Mhz motherboard speed (aka frontside bus). This is the type of RAM you should buy provided your motherboard accepts SDRAM (which 99% of all new boards do) even if you intend to purchase a CPU that requires only 66Mhz on the motherboard. By doing this you make it easier to upgrade later and there is virtually no cost penalty now.
DD SDRAM
This is a new type of SDRAM that is twice as fast as the normal SDRAM (DDR stands for Double Data Rate). It will help systems run even faster. If the price premium isn't too high, it may be worth considering a system based on DDR SDRAM. It will certainly become standard in the future.
RDRAM
RAMBUS RAM is a very new type of RAM that is stupidly expensive and yet doesn't provide any noticeable performance gain. It is used with Pentium 4 motherboards (another reason to avoid the Pentium 4 for now).
EDO RAM
This is an older type of RAM that was launched around the same time as the Pentium 120. It has a 72 pin connection to the motherboard and must be installed in equal-size pairs. You should only need to purchase EDO RAM if you have an old system that supports it (check your manual). EDO RAM is very expensive these days and you may even find it cheaper to buy a new motherboard with SDRAM than to add EDO RAM to your current board!Beware that some motherboards support BOTH EDO and SDRAM - it is however, a bad idea to install both types of RAM in the same motherboard. The reason for this lies in their voltage requirements - EDO needs 5 volts, while SDRAM uses 3.3 volts. If you install both at once, the motherboard will default to 5 volts, thus damaging the SDRAM after a while.
Digital Memory

Any Laptop Memory SD 66/100/133, DDR 266/333/400,
How much RAM?
The answer here is fairly clear-cut. The minimum you should ever get is 64 Megabytes, while 128 Megabytes is highly recommended (256MB for Windows 2000 is a good idea). A machine with a 600Mhz processor and 128 MB RAM will outperform a machine with a 1200Mhz processor and just 64 MB RAM - in other words, limiting your RAM seriously degrades your overall system performance. For 95% of users, upgrading beyond 256MB currently makes no sense in that they will hardly notice any performance increase.A quick word of warning to all those with older motherboards based on Intel VX and TX chipsets (these machines are typically Pentiums running at 90 to 200Mhz). These chipsets do not support more than 64MB RAM cacheable. What this means is that installing more than 64MB of RAM will slow your machine down by about 10% for most tasks - it will, of course, still be faster for memory-intensive operations.