I don't know what the most horsepower made out of our engine is (the engine code is KA24DE for all Altimas, for future reference), and I'm not sure anyone knows since most of us who mod Altimas don't have unlimited funds and no one's really done a "race" project Alty. I'd imagine, going by what I've seen on a 240SX (US-spec 240's have the same engine as the Altima) they could handle around 250 plus some on stock internals. That's probably around 9-12 lbs of boost.
There are several bolt-on mods for Altimas, including pulleys, intakes, exhausts, and a turbo kit made by F-max (not entirely bolt-on, but close). If you get serious about turbo, with some upgraded internals (pistons, connecting rods, etc) you could probably run around 20 lbs of boost and make well over 300hp. The biggest issue is going to be the Altima's "weakest link", its transmission.