Do you have a 5 speed or an automatic tranny? Honestly man, altima's (or at least the first and second generation ones, like the one you have) don't respond wonderfully to upgrades; as a matter of fact, when i put an after market intake in, it didn't perform better at all. An exhaust is going to decrease the pressure and may probably result in LESS low end power. In other words it'll just make it noisy
and your fuel economy will most likely depreciate.