I have a P4 2.4GHz laptop with a SIS 650 mobo. There are two 200 pin slots for DDR PC2100 memory. At present i have 256MB sticks in each slot. I would like to upgrade. Can replace just one of the 256MB sticks for a 512MB stick or must I replace both. What I am basically asking is this - must both sticks be the same MB? What is the best memory and who supplies good memory at good prices. I live in the UK.