What gearing should I use for climbing?

A 27 cassette is not a bad idea especially if you are climbing upwards of 10% grades for a significant period of time. Riding with a 53/39, and a cassette that has 10 gears, 12-25 is what many riders use, however changing your cassette will certainly make a difference in your climbing. A cadence around 50 is particularly slow and so if your cadence is dropping below 60 you may consider getting a 27 if you wish to increase your cadence. Low cadences primarily use fast twitch muscles, which tend to fatigue earlier. A higher cadence will require less force per pedal stroke resulting in the greater reliance of slow twitch muscle fibers, which are more fatigue resistant. Using a higher cadence (>80) will be useful for longer climbs and will conserve energy in the long run. Unfortunately you cannot swap out the 25 for the 27. A cassette with gearing of 12-27 is typical for climbing. You will have to put a completely different cassette on your wheel.