depends on what you do. sa akin, kulang pa ang current na 6GB ram ko sa dami ng gingawa ko. in fact, medyo nabibitin nga ang OS ko after a few hours (i.e. on ng mga 6pm then iwan ko mga 12mn - pag umaga problemado na), explorer.exe hangs, i cant run a new browser window, windows gadgets crashes, very few apps are able to open (except those that are already open), i can execute a restart since the options menu for shutdown,restart,logoff cannot load anymore (shutdown lang nakikitang option), etc etc
first off, does your OS support more than 4GB (i.e. dapat 64 bit)
second, heavy multi tasker ka ba? more apps eat more resources so larger ram helps
third, do you keep your computer on for long periods of time? as time goes by, the ram consumed by the running os is eaten up and it doesn't refresh until the os is rebooted so having a larger ram can help you stay longer without having to restart