|Is your Windows 7 version 32 bit or 64 bit ?|
If it's 32 bit, the operating system can't use more than 4gb, and even then not all of the 4gb can be used.
The 4gb virtual memory address limit for 32 bit operating systems.
An example of 3gb working better than 4gb in a 32 bit operating system.
See Response 6:
If you install 1 or 3 modules in the 4 slots, all the ram will run in single channel mode.
In the real world, installing all the ram in matched pairs in the proper slots so they're all in dual channel mode only results in a tiny performance increase (each pair has to have identical modules of the same size, but they don't all have to be the same size).
It's a lot more important how much total ram is installed, and which ram you use.
Each memory slot can hold DDR2 PC2-5300 (667mhz) , or DDR2 PC2-6400 (800mhz), or DDR2 PC2-8500 (1,066mhz) ram, with a maximum of 2GB per slot.
What PC2-xxxx rating or mhz rating do the original modules, and the new 2gb modules have?
"....mismatch might cause problems...."
What are the part numbers of the original modules, and the new 2gb modules ?