I'm buying a used '04 Trek 5200, stock double crank setup. That means it should have a Shimano Ultegra 12-25, 9 speed cassette.

I also see an ad for a "Dura-Ace TT/Tri 42 tooth chainring". I'm interested in making the hills around here easier.

I don't know much about this, being fairly new to cycling. Would the Dura-Ace be compatible, or should I look for an Ultegra cassette? Are all the gears on the cassette correspondingly higher-ratioed, or maybe just the first few? Any other things I should be aware of?

Thanks,
Paul