I upgraded the hard drive from my 15" MacBook Pro (2.16Ghz) to a 160Gb Intel X25-M SSD yesterday. It has completely overhauled my computing experience. Applications open nearly instantly. Big fatty apps that eat a lot of memory return the desktop to full functionality as soon as you quit. Loading any files is nearly instantaneous. The giant dog of a proggy, Mail.app, now loads instantly.
Ironically, the last computer I owned that was this responsive was my 1MHz Apple //e with a 5Mb Sider hard drive.
At just under $500, it's not cheap, but it is a much more dramatic improvement on my computer's overall speed than any big upgrade I've ever done, and at thousands of dollars less. I recommend it to anyone.