This page covers only the standard RAM types used in standard PCs from the beginning till about 2003, not for example the special types used in some PS/2 models or the RAM used in notebooks.
This is the oldest type of RAM, used in XTs and most 286 mainboards. Usually there will be four banks of nine ICs each, the ninth being for parity check. Since 8086 and 286 processors are 16-bit, these banks are to be seen as two pairs. DIPPs are available as 64k, 256k, and 1Mb, but the latter are very rare. In a 640k arrangement, two banks will be equipped with 256k DIPPs and two with 64ks. You can easily create one 286 with 1MB RAM out of two with 640k, though you have to be careful not to damage all these ICs. Theoretically, a DIPP mainboard with four banks could be upgraded to 4MB, but as I said, 1Mb DIPPs are rare.
These are basically nine small DIPPs soldered to a little board, forming one bank. SIPPs are rare. I have one 286 equipped with four 1MB SIPPs; I have two extra 1MB SIPPs I found in a second-hand computer store once; I have one 386SX mainboard with DIPP and SIPP sockets, but it was equipped with DIPPs. I have no idea whether there were other values beside 1MB.
There were adapters that let you put SIMMs in DIPP sockets, but I have never seen one in real life.
They were standard fare on 386 and 486DX mainboards. A SIMM is basically the same thing as a SIPP, but the contacts are different. SIMMs are less easy to damage. SIMMs come with 256k, 1M, 4M, and 16MB. mainboards usually have eight slots forming two banks of four (386s are 32-bit processors) and supporting up to 64MB. Unfortunately, 4M and especially 16M 30-pin SIMMs are extremely rare, and if they do turn up, ridiculously expensive. So you will usually be able to give your 386 8MB RAM, but no more. The fastest processor I have seen on a mainboard was a 486DX2-50.
These SIMMs were used in many Apple Macintosh computers, too, and here, those with higher values are far more common. Unfortunately, you cannot use Apple RAM in a PC; it lacks the parity chip, and the SIMMs with higher values are thicker than their PC counterparts.
They were first introduced by IBM in their PS/2 series and are therefore often known as PS/2 RAM. Outside the PS/2 series you will rarely find them on 386 mainboards; but they were on most 486DX2-66 and DX4 boards, and standard with the Pentium. A 72-pin SIMM is equal to a whole bank of 40-pin SIMMs, but on a Pentium board you have to pair them again, because Pentiums use 64-bit memory addressing. 72-pin SIMMs were available with 256k, 1M, 4M, 8M, 16M, 32M, 64M, and 128M. The last two are very rare.
Their main advantage is their availability. They are not all that faster. But it easy to upgrade a mainboard to 64 or 128MB of 72-pin SIMM RAM, and it isn't very expensive either. 32MB SIMMs usually just cost the same as two 16MB SIMMs.
72-pin SIMMs come in two varieties, FPM (Fast Page Mode) and EDO (Extended Data Out). EDO is a lot faster and should be preferred whenever possible, but may not be supported by older mainboards. Unfortunately, it's not possible to tell from the outside which type your SIMM module is unless some friendly soul put a sticker on.
When I first wrote this page (May 2003), SDRAM was still widely used. Two years later, it is more or less obsolete and you won't find any new mainboards that support it. It was introduced at about the same time as the Pentium II and ATX boards. SDRAM chips are 64-bit and need not be paired. Nowadays they are available up to 512MB. SDRAM comes in three speeds:
Pentium II mainboards usually have three or four slots and are limited to 128MB per slot, though you can use 256MB chips as long as the total is not exceeded. Keep in mind that Windows 95/98 probably has no use for more than 256MB anyway.
The performance boost compared to SDRAM is not exciting, but once again, DDR-RAM comes in higher units. As of spring 2005, the largest chips are 1GB, but they are still significantly more expensive per MB than the smaller ones. Mainboards usually support 1GB per slot. So we are actually approaching the 4GB limit dating back to the 386.
An increasing number of mainboards support dual channel DDR RAM, which was first introduced in early 2003 and gives a significant speed boost. For this, you need two identical chips. Many vendors sell pairs, of course at a higher price, or you can just buy two of the same brand and type and hope for the best. If your mainboard has three RAM slots, you can only utilize two of them if you want dual channel.