This build is about getting more mileage out of a Nova. Your burst will suffer a bit, but you're going to be able to throw out a lot more abilities a lot quicker, and get AOE damage out of them.

TALENTS:

Psi-Op Rangefinder is going to let you throw out snipes much faster. Also the extra bit of range helps you secure kills or poke from a considerably safer distance, letting you continue to throw out those quick abilities before they can dive you. Headshot is going to stack with this, and also cooldown your other abilities faster, although this is really a snipe-centric build.

Perfect Shot is really necessary when you're decreasing cooldowns. You're going to want to be putting out one snipe after the other, and this will prevent you from having HUGE mana problems.

Explosive Round is going to add a lot of utility to Nova, and combo very well with Railgun. Each target that Railgun hits WILL proc Explosive Round. This adds up to huge amounts of multi-target damage from EACH snipe as long as the enemies are clumped up. Oh, and this also has the nice side bonus of near instant wave-clear at higher levels (although you shouldn't be trying to do this as Nova, it's nice in a pinch.)

Triple Tap with its level 20 Fast Reload is your finishing tool. You should be able to spam enough snipes to single out any enemy you want for death, and as long as they don't have tanks to soak your triple tap, they're history, then rinse wash and repeat with your almost negligible cooldowns on snipe and no cooldown on triple tap. This is playmaker stuff here if you position correctly and use triple tap smartly (patience truly is key once you pick that level 20 talent.)

The one alternate talent I might pick with this build would be Advanced Cloaking at tier 5. If the enemy team is not sticking together so that you're getting single kills and not having another target to engage, or if they're so tanky you're having troubles securing kills before disengaging, the cloaking will help you sustain for health, and also get between lanes even faster (this will help with ganks.)

ABILITIES:

With this build, you're going to be primarily using Snipe and auto-attacks. Pinning Shot is mostly going to be used after your first snipe, to keep the enemy in range for additional snipes, or to help slow an enemy as they try to disengage. Holo Decoy can be used either as misdirection right before you engage, or to juke during an escape.

This build is more of the stereotypical burst Nova build. This is going to rely on you engaging fast and hard, with a specific combo.

ABILITIES AND TALENTS:

You're going to open with a Pinning Shot once you get Crippling Shot which will make the enemy vulnerable to the combo.

Next you're going to immediately throw out a Snipe so that Ambush Snipe still triggers.

After the Snipe, you'll do an auto-attack, which is buffed by Anti-Armor Shells while still stacking with Vulnerable.

While your auto-attack is on cooldown due to AA Shells, you can throw out the decoy at max range so it can pelt them with its own abilities at 25% of your base damage.

Ideally you won't be dying, so Gathering Power will be steadily increasing your damage here. (If you find yourself dying, Envenom would be the next best talent, and can help secure a kill if the enemy makes it out of your range with that DOT still on it.)

Triple Tap here is obviously to secure kills after you pelt them with that ability combo (assuming they're still standing,) and Fast Reload so you keep on trucking during team fights.