For most notebooks, the greatest improvement is going to be upgrading a slow 4200 rpm hard drive to a 5400rpm one (50% or so faster) or 7200rpm hard drive (about another 20% improvement).

More RAM is also a big improvement but above 512mb the gain will be much less unless using Photoshop or another program that actually will benefit from 1mb of RAM. You are limited by the RAM speed by the bios/motherboard/chipset.

A cpu upgrade where possible will have some improvement but the other 2 items above will have more of a benefit.