|"Dual-channel mode can improve performance a little bit"|
Running pairs of modules of the same capacity in dual channel mode is just another one of the things that is much better in theory than it is in the real world.
You are unlikely to be able to tell the difference between the ram running in single channel mode vs. dual channel mode, mostly because it only makes a difference when a program actually benefits from more bandwidth - a higher max data transfer speed - being available, and is actually using it. Most of the time, your computer is NOT using anywhere near the max bandwidth the ram is capable of using.
All the installed modules must be running in dual channel mode, otherwise all of them run in single channel mode,
The ram pairs of the same capacity running in dual channel mode can be matched pairs, which ensures they're identical, or two identical modules, e.g. that have exactly the same part number.
"PS: The manual notes that you may have issues with 1GB modules:
- if the memory chips on the RAM stick are 128MB each
- if they have 16 memory chips and are on both sides of the stick (8 chips per side)."
You can avoid buying the wrong ram modules by NOT buying it randomly.
If you use your mboard make and model to search with on a ram manufacturer's or ram distibutor's web site to look up which ram is listed for your model (works in your model; is compatible with your model) you can't go wrong.
If it's brand name ram, once you know which module part numbers are compatible with your mboard model, you can buy them from anywhere that has those part numbers.
If you already have ram mof the right type e.g. DDR2, it does no harm to try using it, but it MAY not be recognized properly. If it's not recognized properly, there's probably NOTHING wrong with it - it will work and be recoginized fine in a mboard it IS compatible with.
If it's brand name ram, you can look up the ram for that brand to see if the part number of the ram you have is listed.