Human, Rog7Ftr2... first level rogue, levels 2-3 fighter, levels 4-9 rogue
Bump dex to 20, or if you want the skill points, put one in int and one in dex... you can be the party rogue if you want to work that into your concept as well.
Feats:
Human: Point Blank Shot
Level1: Precise Shot
Fighter1: Rapid Reload
Level3: Rapid Shot
Fighter2: Weapon Focus (Light Crossbow)
Level6: Crossbow Sniper (PHB II)
Level9: Telling Blow (PHB II)
Take Improved Critical at level 12. Take Rogue levels the rest of the way, unless your DM consistantly throws you against enemies immune to sneak attack or with very high AC... then consider more fighter levels for the base attack and further feats.
You can always add keen to the crossbow if you don't want to spend the feat slot... just depends on what you are more short on, feats, or coin.