What RAM size in the laptop is the best value for money?

Depends on individual usage. For normal use, 2GB RAM is enough. If multi-tasks and games are top priorities, 3GB RAM best, 4GB better but not really neccessary. It depends on the OS as well. Win XP supports only 3GB RAM so even if you have 4GB like me, it'll only recognise 3GB. Win Vista supports 4GB but it must have SP1 installed. Mac OS X supports 4GB.