34-42 is a real inexpensive solution and middle rings will work inside, just as middle rings will work outside. so for the price of the 34 you could be set. you'll want to lower the front derailleur a bit and you may need to shorten the chain.

as for halfstep... i run 24-39-42 and love it.

one last note, you could buy a triplizer 34 tooth ring that would later convert your crank to a triple. then with 24-34-42 you'd be good for about anything.