Okay, I figured it out.
I opened up the computer, took out the RAM and wrote down ALL of the numbers. I even wrote down the numbers on the board itself underneath the RAM sticks.
When you go to the Bios, it does not say the motherboard number.
In system information, it says that my computer is a "D600".
But I just used a software program to test the hardware.
The software puts my computer at a C600 instead.
And this makes more sense, since the computer manual for the C600 coincides with the numbers and writing on the RAM sticks, and explains why the D600 DDR2 RAM upgrade does not fit!!!!!!!
I looked on the RAM stick so I know that it is SDRAM and 144 pin, but that is all that I really know.
What type of RAM do I use for the Dell Latitude c600? Is it DDR 3? And I want 100 MHz?
If I was going to look for new RAM to get, what are the KEY things that I want to look for to make sure that it is matched up correctly?
Example: [So-Dimm, DD2, #MHz] = key vital things to match for getting "correct" RAM