The most important thing you can do to speed up your Mac and reduce crashes is to be sure it contains enough RAM. Few new Macs ship with what I consider "enough." Even though Mac OS X and most applications can run in as little as 512 MB, in my experience performance degrades quickly with that amount of RAM when you have numerous applications and windows open at once.
How much RAM should you have? The answer depends on the type of Mac, the way you work, and your budget. On the one hand, I think everyone should have at least 1 GB (or the maximum their machine supports, if less). On the other hand, more isn't necessarily better. For example, only a few people running the most memory-intensive applications would benefit from putting the maximum of 16 GB in a Power Mac G5.
In general, if your Mac can hold 2 GB or less, I recommend maxing it out (budget permitting). Go above 2 GB if you spend all day working with heavy-duty photo, video, or audio applications, if you run high-end scientific software that performs complex mathematical operations, or if your computer functions as a server in a high-demand environment.