Gears?

All you really need to buy is the crank, and probably a new rear deraileur (I've never done this though, 2 chainrings is more than enough for me). Just go to a bike shop or look online for a "triple."

This setup actually won't give you any bigger gears to "go faster" on the downhills, the size of the big chainring will probably stay the same. You will however get a small ring that will make it easier to get up steep hills.

RyBike said:

All you really need to buy is the crank, and probably a new rear deraileur (I've never done this though, 2 chainrings is more than enough for me). Just go to a bike shop or look online for a "triple."

This setup actually won't give you any bigger gears to "go faster" on the downhills, the size of the big chainring will probably stay the same. You will however get a small ring that will make it easier to get up steep hills.

Sorry! rear deraileur? why!

you would need a chain drive (crank) a font mech suitable for triple and possibly a bottom bracket to suit the cranks that you have changed to depending what the length of the axel was, on the understanding that the one that you are using now was either to long or to short, you could always try purchasing something like a 56 tooth chain ring for your double, or user a smaller gear and spin faster.