This is straight from the User's guide for the C540/C640
Memory
Memory module connector
two user-accessible SODIMM sockets
Memory module capacities
128, 256, and 512 MB
Memory type
266-MHz DDR SDRAM
Standard memory
128 MB
Maximum memory
1 GB
I have this exact Laptop and It holds a maximum of 1GB of memory. It has 2 memory banks. So the largest single sodimm you can put in it is 512mb. If one of your memory banks is broken then the max memory you can install is 512mb. Sorry.