|You need to have three matching sticks of RAM installed to get the maximum out of the system.|
Dual Channel or Triple Channel RAM is not special RAM. It is modules that are exact matches of one another. If you separated them and used them in three different systems they would run just like any other RAM. The memory controllers are what is triple. May be on the board or in the processor. Either way an analogy would be a three lane highway where all the lanes go the same direction. That highway can carry 3 times the cars (data) as a one lane highway with the same speed limit. Dual channel is a two lane highway.
Get the idea? The only reason for buying in a kit is to assure the RAM modules are exactly the same. At the speeds modern RAM runs slight differences could cause errors or slow down the entire system.
The reason jam advised you to get the 6GB kit is because you have two choices, basically, 3GB or 6GB. If you got the 3GB kit and needed more RAM later (Windows 7 64bit) you would need to pull the 3GB kit out. That is not cost effective.
Extra RAM doesn't hurt.
The cost isn't double.
You are already spending a small fortune on the new build anyway.