Hey Ubuntu users!

I recently bought a new laptop because I wanted to get into Ubuntu, but I'm having a problem. I originally installed 32bit and didn't realize until later that I would need 64bit to fully support my 6GBs of RAM. After going through the installation process again, this time with 64bit, the 6GB of RAM just isn't showing up -- or at least not all the time. The weird thing is that every now and then, when I check under "About This Computer," it shows 5.5Gig of RAM (which is normal). Also, when I hold down F2 on boot-up it shows that 6GBs of RAM is installed. I tried "free -m" in the Terminal like some suggested but it only shows the 1.5GB (2GB stick) that came with the computer. I tried removing the 2GB stick that came installed but then the computer won't start. I thought that it might be the wrong type of RAM, but if that was so wouldn't BIOS just not show the 4GB stick at all? Also why would Ubuntu recognize it part of the time?