The dagger one is after the bow one. Ur thinking of the one-handed perk, u can use sword, axe, even a mace. U will get more damage (since they do more damage than a dagger). The dagger perk's name is "Assassin's Blade" or something.
[TABLE="class: wikitable sortable, width: 1"]
[TR]
[TD="colspan: 2"]
Backstab[/TD]
[TD]00058210[/TD]
[TD]30 Sneak[/TD]
[TD]Stealth[/TD]
[TD]Sneak attacks with one-handed weapons now do six times damage.[/TD]
[/TR]
[TR]
[TD="colspan: 2"]
Deadly Aim[/TD]
[TD]001036f0[/TD]
[TD]40 Sneak[/TD]
[TD]Backstab[/TD]
[TD]Sneak attacks with bows now do three times damage.[/TD]
[/TR]
[TR]
[TD="colspan: 2"]
Assassin's Blade[/TD]
[TD]00058211[/TD]
[TD]50 Sneak[/TD]
[TD]Deadly Aim[/TD]
[TD]Sneak attacks with daggers now do a total of fifteen times normal damage.
[/TD]
[/TR]
[/TABLE]
http://www.uesp.net/wiki/Skyrim:Sneak