Well overclocking will not void your warranty because Apple has no way of knowing if a card's been overclocked or not... unless of course you flash and by the look of things it seems like flashing will be your only choice, there was some application which allowed you to dump the rom, write the new clockspeeds to it and reflash your card, well actually if you keep the original rom and reflash it should you ever need to apply for warranty then all will be fine.
I don't know of any programs for mac but there are probably some, I remember there was a program for nvidia cards but there is probably one for ATI cards too, you can always do the flashing from windows though... (the card will work in OSX)
The risks are minimal, you won't fry your computer or your videocard, at least not from a minimal overclock like that, however if you notice any signs of instability, graphical glitches or anomalies (artifacts) or crashes\hangups\kernel errors then return the card to stock speeds, a good way to test stability is to run long benchmarks of an intensive videogame, however, with such a minimal overclock I don't think any testing will be needed.
The big risk would be messing up the flashing process, flashing is very easy however if something goes wrong then you can effectively render your card useless however I've never messed up a flash so I don't know the real consequences.