You have 256 Mb in one socket, and one empty socket.
256 MB RAM is really not enough, you should definitely think of another 256, or even better a 512 Mb or 1 Gb module. The machine will take a maximum of 2 Gb RAM if you use 1 Gb modules.
In particular, you will need PC2700 SODIMMs. The Kingston part is M6464C250 for the 512 Mb. In the USA, I recommend this RAM from Data Memory Systems http://www.datamem.com/parts_that_fi..._E_Acer_A_C_E_
$59 for 512, $144 for 1 Gb