As quoted from pcstats:
Generally speaking more computer memory is better than less. However, installing more than 2GB of RAM doesn't necessarily translate into a faster Vista desktop PC. Only a few applications may see improvement, and then the law of diminishing returns kicks in. So 2GB of RAM is generally considered the sweet spot for Windows Vista. As the operating system matures, along with supporting software, it's possible that number will climb. It's a wise move to choose a PC that can be upgraded to 4GB.