In order of impact:
- I just dropped a cheap 60gb solid-state drive in a 4 year old computer, and it runs faster than a lot of new ones out there. Mac or PC, desktop or laptop, you owe it to yourself to get one, and they start under $100.
- Read the first point again: regardless of processor speed or age, a semi-modern machine with an SSD will feel way faster than one with a hard drive.
- If you have a mac, consider replacing your optical drive with an SSD. Seriously, when was the last time you put a CD in your computer? I did this a year ago, and I love it: my OS and apps live on the SSD, and my media lives on the hard drive.
- Upgrading your RAM is subject to diminishing returns. You need 4GB if all you run is browsers, Office, and video, and 8GB if you're heavy into Photoshop or virtual machines.
- If a Windows PC feels slow, back up your documents and do a clean install of Windows 7. On anything made after 2005 or so, it's faster than XP (just turn off Aero if your graphics card sucks). I just deployed several old Lenovo laptops in my classroom, and they run great with Windows 7.
Keeping a bunch of space free on your hard drive is at least as important as defragmenting it (Linux and OS X handle defragmenting automatically). Shoot for 20-30% free space if possible. Unlike hard drives, SSD's don't slow much as they fill up.
Update: read this awesome, nerdy Engadget guide to the history and big-deal-ness of SSDs.