10.6 and future versions of MacOS X won't run on a PowerPC (he has a G4). So that alone is a reason to upgrade to a newer Intel-based machine.
However, if your computer is working well for you, you should always question the "need" to upgrade. We've gotten to the point where even older computers are perfectly capable of handling huge files in Photoshop, etc.
I have a friend who still uses a Powerbook from around 2001. It works perfectly well for web browsing, downloading files, watching videos, etc. He's running 10.5.
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