I have a Lenovo 3000 K100. It had 3 DDR2 - 1GB memory sticks from the factory which were PC2-5300 sticks. The mother board has 4 slots and I put in 4 - 2GB sticks which a friend had available. They are PC2-6400. When I run the Crucial scanner I see that they are recognized. My computer seems to be running fine and faster. Should I expect any problems? I did notice that one of the sticks is rated at 800Mhz rather than the 666 or 667 of the specs which are called for. I also noticed that when I run Task Manager it shows that my physical memory is only 3061 MB. Any good advice will be appreciated.
You will probably be Ok using the other memory, but I would keep your original memory for a while just to make sure there are no issues. 800MHz (PC2-6400) memory is usually an acceptable option.
As for Windows seeing only 3GB of RAM this is a limitation of 32bit Windows.
When your system becomes too slow due to the low memory, I would suggest trying Linux on it as it does very well with just 4GB if you are willing to take the time to learn how it works and how to use it though it is not for everyone.