You usually have a lot of processes running that you don't need(adware, media agents, etc).From the task manager find out what processes you need(windows shell, system processes, video/audio drivers) and the ones you don't.They all consume resources, and usually don't all need to be running.Also remove apps from the startup folder that don't need to be ran on bootup.Try a program called MaxMem from
www.AnalogX.com , this will flush the mem at certain intervals and helps alot, also various XP tweaks from downloads.com will help a ton.Adding extra ram wouldn't hurt , but I don't think it will help with apps such as Outlook.