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Yes, that's right, 1 GB and 256 MB. But the performance with two modules was much better; there were no freezes at all. And now, it freezes every 5 seconds.
I would buy another 1 GB module now, but I'm worried that the second slot had contact issues before, when two 256 MB modules were installed. If the motherboard is faulty, it's better to upgrade the entire computer.