It's not essential to have sdram in pairs, so it should work as long as the new ram is compatible with the mobo.
Have you checked the existing ram to see if it's 2x128Mb sticks, or a single 256Mb stick? If it's 2 sticks, you may need to remove one and just have 640Mb (most laptops have only 2 slots).
If it doesn't work, try the 512Mb stick on it's own. If that doesn't work, it's probably not compatible, or you may need to alter the ram settings in the BIOS, if that's possible.