The Card Sharp talent allows the rogue to gain the Deadly Dealer feat, even if they do not otherwise qualify by having Arcane Strike and 5 ranks in Sleight of Hand. Deadly Dealer allows a rogue to treat a card as a dart, which is a piercing, ranged weapon with a range increment of 20 feet that deals 1d3/d4 damage for Small and Medium builds, respectively.
Sneak Attacks are allowed to rogues whenever they are within 30 feet of range so they can spot a vital part of their target's body (the face/throat you mentioned), and at least one of:
- the target must be flanked by the rogue or
- the target must otherwise be deprived of a dexterity bonus to AC, such as being caught flat-footed during the surprise round
Underhanded allows you maximum sneak attack damage if the opponent is attacked with a concealed weapon (a deck of cards should appear as a mundane item to an average, run-of-the-mill guard and is therefore concealed until revealed to be a weapon during an attack on said guard) during the surprise round. As the surprise round is defined by who is aware of their opponents, only an unsuspecting guard would be an applicable target. Your rogue approaches, aware they want to kill the guard; and the guard must decide if the rogue belongs there, is just part of the passing throng of rabble, or if he's a recognizable enemy. The efficacy of this build seems heavily dependent on the situation and is subject to some GM discretion. Also, you've an initial damage roll range of 7-11 (1d4 + 6(max sneak attack damage due to Underhanded), and only if you hit the target's armor class. The maximum range increment for sneak attacks is 30 feet and the guard, or his partner, will easily close the following round.
Otherwise, it is a fun idea with a lot of storytelling and flavor potential and I think you should go with it. :)