IMO, you are making way more out of this than you need to. Most of us change out cassettes for different locations or whatever. I usually run a 12-27 (double, on my road bike) but sometimes as big as an 11-32 (on my Cross bike). As long as your rear der can handle it, just pick up a 12-25 and see what you need from there. It is only another $30 or so to make a change. I swap cassettes a few times a year, depending on where I am headed or how I am feeling.
IME, gearing is something that just needs to get you close to optimal, not necessarily to the PERFECT gear inch every time.
I agree with the suggestion that you try to go with a rear gear # that is as modern as possible. YOu don't want to end up being one of those poor suckers who are desperately bidding up 8 speed components on ebay because they have no choice (supply versus demand).
Last edited by Sawtooth; 08-20-09 at 12:24 PM.