Restart issue is usually a PSU or mobo problem. But if you have the restart upon error box checked in windows, your not seeing the blue screen. If you have a bad stick of ram its not able to read the cell it wants and will blue screen. (which will then reboot)
You can test this by either using memtest on your ram, or by removing and putting the 256MB stick back in. If the problem goes away, you have a faulty stick of ram.
I've now taken out the 512mb stick and just left the new 1gb in and it's working fine.
Think it has something to do with the mix of memory, but not sure. They are both pc2700 and specs for 1gb were right on the crucial website, so don't understand why it won't work???
if you know the specs of both sticks put them both in and make sure they are set up properly in the bios. Perhaps its not mixing them together properly. You might also want to check each stick separately with CPUz to make sure you have what you think you have.