I would go with what the Asrock site says. That Crucial link Alan had posted is a great link and this is actually the first instance where I have seen it telling someone something which is wrong (unless I am completely wrong here, haha)
The memory you currently have installed is DDR266, so you could add another 1GB stick to your board which will improve performance a lot. The best thing to do would be, depending on how much RAM you will be wanting, is to set it up in dual channel. Either 2x 512MB sticks or 2x 1GB sticks.