A real world example: at work we have a
Power Mac Dual G4 (Mirrored Drive Doors) we purchased in October 2002, at the same time I started working there. It was our department's first Mac, with dual 1.25GHz processors and 1GB of RAM.
The only hardware upgrade we've given it is an additional 200GB hard drive, though we are going to attach a much larger external firewire drive to it soon.
Now, it's nearing the end of January 2008. It's running Leopard and still going strong. Maybe it's not as fast as the 24" Aluminum iMac now sitting next to it, but still rises to the occasion whenever we have it doing audio or image/photo editing work, and it still gets the occasional DVD video project whenever the iMac is busy doing something else.
Contrast that to our PCs: that Power Mac has stayed around while the office has gone through three complete PC upgrade cycles. Though within the past year, our Mac spending has increased significantly, and they're replacing workstations that were typically equipped with PCs in the past. Our end-product is based on a UNIX platform, and it took our IT staff and top brass a long time to recognize that, hey, the guys using Macs are having an easier time of things.
Anyway, our Power Mac doesn't look like it wants to die any time soon. Its CRT display is starting to look fuzzy, but we're going to replace it with a flat panel LCD. We plan on keeping it around at least until the next major release of OS X comes out (probably longer if by chance the hardware is still supported).
Bottom line: a well-equipped Mac will last you a long time. There's this hypersensitivity in the Mac world to always have the latest and greatest, but just because Apple releases something new several times a year does NOT make your existing hardware immediately obsolete. Maybe it won't look as stylish and trendy, but it'll still be plenty useful and will serve you well for a good while.