Join Date: Apr 2008
Bikes: Cervelo RS, Specialized Stumpjumper FSR Pro, Schwinn Typhoon, Nashbar touring, custom steel MTB
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Cheapest thing would be to swap the 43-tooth chainring for a 39-tooth ring, though it doesn't change the gearing all that much.
Compact cranks are a semi-recent invention. Most new ones are designed to work with 9- or 10-speed drivetrains. Dunno what happens if you try to mix a 10-speed compact crank with an older 6-speed drivetrain. Might work, as long as the chain isn't too wide and the chainline is similar.
Switching to a triple would require a new shifter (if your bike has indexed front shifting), new front derailleur, and the new crank. You might also need a new bottom bracket, shift cable, and chain.