You have a laptop which RAM is higher than 3 GB and you use Ubuntu 32bit OS? So go to System Monitor (System->Administration->System Monitor) and select Resource tab. You will see that your RAM is shown less than computer’s RAM. For example my laptop has 4 GB RAM, and it showed me 2.3 GB. I have researched problem and found a solution. 32bit systems can not take all RAM in default. You must install 64 bit or you can stay in your current OS with installing something. This something is Physical Address Extension (PAE). It solves a problem clearly.
For installing PAE go to Terminal and type:
sudo apt-get install linux-generic-pae linux-headers-generic-pae
Now reboot system. Go to System Monitor and you will see that RAM is nearly 4 GB, yeah.
That’s all. Good luck.