I heard that as long as you add NOS, it makes the fuel colder, and then you won't need forged internals.. Of course, you have to make sure that you use both at the same time!! Just NOS or Just a turbo alone and you still have to forge!! That's only because the turbo compresses the NOS and then it gets RALLY RALLY cold!!
I'd say about a 200 shot and then you could go maybe as high as 30 pounds without needing to get forged... but ONLY if you do it with a 200 shot nothing less!!
Then, again, that's debatable too!
Oh yeah, it has to be NOS brand!! If you use a Zex or NX kit probably better make it a 250 shot!
OK... All BS aside (sorry but I couldn't resist).
The general rule of thumb for a stock 4.6L 3V motor is that Below 10PSI or below 500rwhp is generally considered the "safe zone" before you have to start looking into a forged block. Keep in mind though that 'safe' is a relative term and going forged is the only true 'safe' bet. But there are many mustangs out there (mine included) that do just fine on low levels of boost (as in under 10psi).
On the other hand it depends on how lucky you are too. There are engines that let get with as little as 5psi while others get driven daily running 12-14 pounds on stock internals. It really just depends and a lot of it has to do with getting the right tune on the car.
Even with low levels of boost if you go lean you can easily hole a piston or worse. Intercooling also plays a gigantic role on how far you can push it. I wouldn't go over 5 pounds non-intercooled.
I run a 5psi blower on my car and drive it like I stole it. So far it's been trouble free except for one instance where I had a bad ground on one of the Fuel Pump Driver Modules but that wasn't related to the engine. Also I've got a very safe tune on the car. I pull steady 11.0 - 11.2:1 Air/Fuel ratios while I'm in the boost which helps cool and lubricate pistons as well as avoid detonation.
Again, as long as you're below the 10psi/500rwhp mark, it's 60% about having the right tune, another 25% about your personal driving habits, and another 10% about getting a motor that wasn't built on Monday (if you catch my drift)