Dell Inspiron Motherboard/Memory Problem

[Jaco42]

So I have a Dell Inspiron (laptop) 5100 and its been working fine for 4 years with the exceptions a some overheating issues. Well recently, I closed it while it was still running and when I opened it 2 hours later the screen was frozen. As soon as I trying to turn it on again it shut off after 3 seconds. It would begin to boot up and while the screen was still blank it made a short blinking noise and shut down. After spending awhile with Dell tech support, I learned that as long as I only have one of my 256 mb sticks of memory in the slot the computer will run. It doesn't matter which stick I use but the computer will run with only one 256 stick of memory. Either slot works. Strangely, I tried both sticks once and it got to the XP welcome screen only to make very strange noises and shut down. Dell thought it was the Mother Board but I have had some bad experiences with listening to tech support people who read off scripts. I bought a half gig of memory stick hoping that will just replace the two 256 sticks. Do any of you know what the problem is and if buying a 512 mb stick will work? Is it the motherboard or something else?