Simple. Look at the specs of one that is 5.5 years old right now. That would be a January 2004 model. Can you see yourself using that today? Is it powerful enough to handle your intended usage? and to run the applications you need? That will answer your query.
WHile I agree with the consensus that a mac can last you six years (after all i still use my G3 ibook from 2000 and my Powerbook from 2005!), however regrettably things aren't nearly as simple as this today.
Back with the PPC macs, upgrades were particularly slow, sometimes with only mild 0.2 Ghz processor upgrades in an entire year. With this limitation (caused by the slow development of PPC chips due to their heat and power restrictions), apple were to some extent forced to refine their OS to run more and more streamlined, ending (to my mind) with OS 10.3 or panther, an OS that seemed to sip from the computers processors and ram, rather than guzzle it down! In fact, it was so streamlined that it is recommended above earlier versions for macs built years before its introduction - no simple feat.
Now we are with intel, what is great is that the speeds of the machines we use are really increasing (even if sometimes it doesn't seem so), with upgrades far more often, and ones that provide significant progress with CPU, RAM speed and FSB etc.
The downside therefore of these faster macs is that unfortunately they don't tend to last as long as the old PPC ones.
However (and finally!), Snow leopard should be particularly useful in this regard, and also it should not be forgotten that no matter what any nay-sayers here may attempt to convince you, a mac today will perform the as good in six years as it does today if using the same programs. The only problem comes with innovations that bring new functionality over time (such as core image, coverflow in itunes etc etc).
If you are happy with the functionality now, and furthermore aren't someone that needs the latest and greatest (as many here seem to be), you will be absolutely fine speed wise over the next six years. Just understand that any equipment, computer based or otherwise, can break at any time.
🙂