I've overclocked a 9800 Pro before. It didn't go very high above stock speeds before losing stability, but I think I had it running at least a little above normal just fine. Default clock speed for the 9800 Pro is 380 MHz core, 680 MHz DDR. I got mine to 400/700 or something like that. I didn't see any change in performance, though.
Technically it would void your warranty, but the only way that would come into play is if you actually wrecked your graphics card doing this. If you set it back when you're done and you don't do any damage, no one will be able to tell. And damage will only result if you leave it running at too high a clock speed for too long. Generally, the computer will shut down or you'll see artifacts before it starts running so hot as to cause damage. The way I see it, it's just kind of a hobby-like thing you can do, and just set it back once you've messed around with it. At least with the 9800 Pro it is, as they don't overclock much.
If you have a lower-performing Mac, especially one with a 9600 Pro, then I'd suggest trying it out. A friend of mine got a nearly 100 MHz overclock out of a fanless Apple 9600 Pro. Should also work pretty well with eMacs, and maybe even iBooks. The Radeon 9200 is a cheap, low end card with a low clock speed so I wouldn't be surprised if it gets a much better overclock than a higher end card like the 9800 Pro.
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