going double to triple

[SadieKate]

The rule of thumb I've been given is 10-12 tooth max difference between rings for the snappiest shifting. This is part of the reason there was a real lapse in the performance of shifting on compact doubles, the 16 tooth jump. Until recently many front derailleurs just weren't designed for this gap, hence the intro of derailleurs designed for compact doubles.

[Eden]

I have doubts that a 28 tooth difference would shift at all.... on my old bike with a triple it was *hard* to shift from the 30 to the 42 and that's only a 12 tooth difference.

[SadieKate]

I went back and noticed that the drivetrain is Campy. Definitely stick with 10-12 tooth gaps max.

Look at a nice polished Sugino triple with 46-36-26. This should give you a nice set of climbing gears while still maintaining the tight cogs set for flat land riding. You will need a new bottom bracket but you can get some nice quality JIS taper BB's for a reasonable price.
http://www.sheldonbrown.com/harris/cranks/11074.html

For upscale cranks, you can look at TA cranks from Peter White and customize the ring choices even more.
http://www.peterwhitecycles.com/carmina.asp

Here is info on calculating your derailleur capacity
Chain Capacity and Cage Length
http://branfordbike.com/page.cfm?pageID=62

[SadieKate]

I have doubts that a 28 tooth difference would shift at all.... on my old bike with a triple it was *hard* to shift from the 30 to the 42 and that's only a 12 tooth difference.

Yeah, typically the smallest ring to the middle ring should only be 10 while you can go to a 12T gap between middle and big.