This will occur if you have Dual Chanel RAM and you fit a third stick your system will be forced to run in Single Chanel Mode so the FSB of the CPU will be cut in half.
If you where to add 2 additional 512 MEG Sticks this wouldn't be an issue but adding 1 additional Stick may cause the system to not run to the best performance possible.
It really all depends on the hardware involved here which wasn't made mention of in either this or the previous question.
If your system runs with Single Chanel RAM there will be no difference in speed of the FSB for the CPU but the hardware would have to be fairly old for this not to come into play.
The difference here is that Windows will happily support 3 GIG of RAM but the Hardware required 2 lots of 2 Sticks of RAM to maintain the Dual Chanel RAM that it uses for best performance.
As for fitting the RAM open the case and insert it into the Socket. Just remember not to touch the old contacts on the RAM and to touch the case before reaching inside it to bring it to the same Static Potential as you are at. Also unplug the Mains wouldn't hurt.
Col