Originally Posted by pocky

Doesn't need to be genuine Shimano. Anything that uses the same 9-speed spacings and splines as Shimano will work fine. SRAM (11-21 and 11-23 both available at Nashbar for $42), IRD, SunRace, store brand, even a cassette cobbled together with sprockets from several different cassettes of different brands -- and even Campagnolo 9-speed spacing is "close enough". The cheaper it is, the more likely you'll be to replace it more often, which will make the biggest difference in performance and longevity.

IMHO, Dura Ace is complete overkill for this application.

And if your big ring / big sprocket combination gets much smaller (as it certainly will if you're moving from a 34 to 21 or 23 for your biggest sprocket), you probably want to shorten your chain.