It also depends on what you're using the laptop for. Other parts are often the bottleneck, for example:
Graphics:
Often the bottleneck when gaming, not the CPU.
Boot times and application loading:
Often the hard drive, so a fast SSD on an older laptop would perform basic tasks better than a slow hard drive with a fast CPU.
If you don't game and want your computer to be simply more responsive you should invest in an SSD. You can easily CLONE the drive. I don't recommend less than 120GB for an SSD (like the OCZ Agility 3) and make sure it's large enough.