I used a 52/42 with 11-32 in back most of this summer. An odd setup, I know, but it seemed to work out pretty well. 'Course I had been using 7 speed previously so the gaps in shifting felt to be about the same. Currently my main ride has a 39 x 26 low gearm which seems adequate. Haven't done any ultra hilly 200 mile races or anything though...

To the OP I'd go for the 28 (as zecannon suggested) myself since you've also go the 34. That's a pretty low gear. Other than that, 32 I guess. The shifting will feel clunky either way compared to a standard road cassette, but that's just the price you pay for a wider gear range.