I got a new 2GB RAM stick and I am going to add it alongside my existing 2GB card making it 2x2GB=4GB RAM.
Do I have to do anything extra to make it work as 4GB fully?
Anything to do with the PAE kernel stuff?? (I already have a PAE kernel)
You shouldn't have to do anything, unless you want to upgrade from a 32bit to 64 bit system and you want to use a 64bit OS.
The only thing that you might have to do is adjust your swap file size. to do that take a look at Swap FAQ
If you already have the 64-bit or PAE kernel; Nope.
Either of these kernels will detect and automatically use all 4GB of memory. The pae kernel is the default for Ubuntu 12.04.
If 4GB RAM is detected via free -m
or through System monitor
and since you already have PAE Kernel installed , then you have to do Nothing .