I have a Lenovo IdeaPad Y510 and I have Ubuntu 12.04 32 bit installed on it. I think my machine does support 64bit OS. Currently I have 1GB RAM and I am planning to upgrade it to 3-4 GB. After upgrading RAM can I install 64bit Ubuntu on my machine(for better performance as 64bit performs better than 32bit)? If yes then what this link is talking about RAM usage. I didn't get it all.
If only for support of more RAM an upgrade to 64-bit is not needed. In case you did a fresh install of 12.04 32-bit (i.e. no upgrade) you will already have a 32-bit pae kernel installed that supports up to 64 GB of RAM.
You can not "upgrade" 32-bit to 64-bit Ubuntu without re-installing the OS. This however may not be too much trouble in case you keep your HOME. If you decide to do that please make a backup of your important data before you proceed.
My personal recommendation in case you are happy with your installation, and you don't need to run 64-bit-only applications, would be to just leave your OS as it is. This is the least hazard to your data.