Most memory modules are compatible to operate together, there is no need to replace the existing module provided that there is a free slot available in the machine you are upgrading to insert the new memory module.

Unless otherwise stated in the system information for your model, all the memory upgrades are compatible with your existing memory.

Alot of machines support "dual channel" these days, however, the real-life performance gains of dual channel are very little. For most people who use it for everyday tasks like email, internet etc., there is no noticeable difference as it does not "stress" the memory subsystem.

Most people like to keep alot of programs open at the same time (e.g. web, emails, iTunes, Excel). This requires alot of physical memory to hold all the programs while the machine is running to minimise "lag" when switching between the programs. Thats why adding *more memory* is always a good thing.

Memory usually receives instructions 1 row after the other. Dual channel as the name implies, splits the instructions and sends them down 2 rows at the same time. This is why you need "matched pairs" of memory so that it is an even split. The reason most people don't see any improvement in speed is because the programs that they use are not supporting this feature. (e.g. your email program just sits there until you starting typing a new email - most of the time your computer is waiting for you!).

Dual channel is great for intensive number crunching like rendering special effects in movies or the back-end servers of banks that process 1000s of internet banking customers per hour). After all, dual channel was originally developed as a server technology but has filtered down to the consumer level over the years.

There are two main reasons why the system properties within Windows does not read the the total RAM installed. Most onboard graphics on both desktop and notebook systems will share the physical memory installed, the amount that is shared ranges from 8MB to 512MB which is dependent on both the onboard graphics and the amount of RAM installed. Some manufacturers do allow you to changed the amount of shared memory within the BIOS setup. 

When installing more then 3GB into PC's running Windows XP and Vista (RTM) 32-bit editions, the system properties will only recognise between 2.75GB to 3.5GB depending on the specifications and hardware in your PC. This is a known limitation of the Microsoft Windows operating system and a possible solution is to replace your current Windows installation with another Windows edition or OS that is able to address more then 4GB of RAM.

For more information read Microsoft Knowledgebase article: http://support.microsoft.com/kb/88813

What if my graphics is discreet (not onboard)? Due to the memory mapping feature within the Windows OS, the Video RAM may be also resevered between the 3GB - 4GB adressing range reducing the total amount of RAM read in the System Properties dialogue box.