Your motherboard supports upto 2 GB RAM. See the essential specs
at this link
If everything's working normally, there's no reason why your mobo should detect only one RAM stick - even if they are rated at different speeds. Is the 256MB stick detected when you turn it on without the 512MB piece?
As your motherboard is fairly old, the problem may be imperfect contacts at the RAM slots, especially if you have been having rainy weather recently. Remove both RAMs, wipe the contact edges with cotton cloth and push them in again firmly, and see if they both get detected.