My do's an don't suggestions
Do: Make a clear distinction between the character of the noble and that of the king. In addition to the clear alignment schism, have them differ on policy, background, class archetype etc.
Do: Have at least a minority of the population definitely helped by the noble.
Do: Have a some sort of connection between the noble and the king, that predates their current situation
Do: Consider having both factions offers the PCs quests.
Do: Have the noble offer alternative solutions to problems that the king is dealing with. Especially ones that include the PCs as part of either solution. (IE you could go with the X's plan of taking on the kobold kingdom, or you could buy their peace with a magic items rumored to be in those old temple ruins...)
Do: Have an agent who either betrays the PCs, is crucial to infiltrrating the secret society, or both.
Don't: Have the noble concerned much more with than power and what he intends on doing with the a power. A doomsday device, genocide, bitter jealousy, or special kind of greed seem to take away from the character sketch.
Don't: Have the PCs discover all the details of the secret society at once (They are after all secret).
Don't: Have the secret society manipulate the PCs in to doing all their bidding