RAM

Clarifications

memory the same.It can cause complications otherwise.Check in your bios to see what is the maximum mhz your motherboard can handel

You can mix sizes, ie: 256mb + 128mb or something like that. Not a problem. If you mix speeds,(*) (and if it works) they will only run at the slower speed. (and no gain there.)
You can only use RAM as described by the computer manufacture (or mother-board manufacture) So in that JC is totally correct.
There are many types of RAM (ie: RIM, EDO, SDRAM, DDRAM, DDR?, ect.) out there, try to do a quick search and get aquainted with a few, for your own information.
(* as for speed it will be classified as a PC-100, or PC-400, or simular, which refers to a speed factor in megahertz (Mz) from 66Mz (old) through 533Mz)

Of RAM on any M'Board and allowing Timing Errors from creeping in and causing problems occurring down the track. If you are just looking at a games computer no big deal but if you are looking at something that is [b]Mission Critical[/b] where the output is required to be accurate I not only prefer to keep the same type but the same Brand & Batch wherever possible.

Col

There is not such hard and fask rule that the motherboard shall accept any type of RAM.
Basically you have to see the mother board manual or specs where they mention the memory columns as SDRAM/RDRAM/DDR/DDR2.
Each and every slot differs, if you see on the motherboard, just beside the RAM slots you can see which type of module it supports like DIMM1 - DDR2. You can not push DDR2 module into SD-RAM slot, do not even try it as it could break the slot.