First off: You really don't need a special build to be good at Sniping in FO:NV. Any weapon with a reasonably good scope and sufficient skill/strength to not have it wobble all over the place will be fine. High luck for increased crit chance is nice, but most of your attacks will be Sneak attacks with a 100% chance to crit anyway. Boone's Spotter and ED-E's Enhanced Sensors bonus perks help a lot as well.

Beyond that, and beyond the usual damage increasing Perks (Better Criticals, et al), you may want to look at perks like Hand Loader, which provide the best ammo you can get for weapons like the Gobi Campaign Scout Rifle (Only available if you can get to 100 Lockpick buffed!), and other high end scoped weapons. The Cowboy perk doesn't work with the best sniping weapons, but there are a few scoped guns suitable for mid-range or mid-game use.

One other interesting possibility is to go an Energy Weapons route, again, taking the usual + Crit and +damage perks, and being sure to grab Meltdown ASAP. Then pick up the YCS/186 Gauss Rifle and snipe things from a mile out, and watch them explode to kill their friends. Fun for the whole family!

This data is taken directly from the GECK.

Chem        Add. chance    Add. effect
--------------------------------------
Hydra       10%            EN -3
Cateye      -              -
Turbo       20%            AG -2
Buffout     10%            EN -1 ST -1
Jet         20%            AG -1 CH -1
Mentats     5%             PE -1 IN -1
Psycho      10%            EN -1 PE -1
Steady      80%            AG -1 ST -1