well, since this is a MBP forum, I assume we are discussing the available options
processor: Available upgrades are small. Only worth it if you need every last percent of power/performance
RAM: Going from 4GB to 8GB is useful in the long term. However apple's upgrade option is horribly overpriced.
Graphics/video: can't be upgraded, unless you count going from 13'' to 15'' an "upgrade." Valuable if you want to game, also for some graphics intensive applications
Hard disc: Upgrading to a SSD is awesome!