Your laptop can handle up to 1GB (1024MB) of RAM. The maximum would be obtained by having TWO modules of 512MB.
You currently have a single 256MB module of RAM.
To test the 512MB module, turn off your laptop completely, then remove the 256MB module and replace it with the 512MB module.
Does this work with just the 512MB module?
If yes, then your module is fine and the problem is either:
a) the second memory slot is faulty, or
b) two sticks must be equal (I don't believe this is so)
FYI, when you have either one stick, or two different sized sticks installed you only get HALF the speed of your memory as with two identical sticks. it may not matter unless your CPU is being held back (probably not).
Summary:
- get a copy of MEMTEST (...