Vista itself is a resource hog, Vista Aero in particular can eat up a ton of resources. Aero is a group of features installed as default in Vista when Vista is installed. This group of features includes visual effects, helpful animations, transparent menu bars and thumbnail previews.

Aero is installed as the default theme when Vista is installed on your system. This default theme hordes allot of valuable resources that could be effecting your system. Disabling this default theme will free up valuable resources, and you still keep the Vista look.

You will not have the 3-D flip option for your desktop if you disable the Aero theme, but you will have the general Vista look still. You either like the 3-D flip option or you dont, its not a necessity to have.