Sudden reboot

The pc seems to be working fine. I am puzzled as to why. I removed a memory stick I added when I bought the unit. Could that cause it to suddenly reboot? It came with 2 sticks of 256 mb. I purchased a stick of 512 mb and installed it. Can this cause such a strange problem? If so how would that cause a reboot? Then the question is how do I resolve the issue? I would like to have 1 gig of memory. he mb has 4 slots and is said to support up to 4 gig. Any help would be greatly appreciated.

Thank you