Ditto with respect to cadence. I would make keeping your ideal cadence your highest priority, and then build your gearing to suit it.

I personally like higher cadences, which dictates using lower gearing. I do 100-120 rpms on the flats and 80-100 on the hills. A 39-26 would give you a speed of about 15 km/h at 80 rpms. What kind of grades you can conquer with your gearing also depends on your strength, fitness, and weight.

To help evaluate your gearing, you may find useful a set of Excel worksheets that I wrote up. You'll find them here: http://www.uni-bonn.de/~jmlee. Please read the instructions.

Cheers,
Jamie