Ziffy's post is pretty accurate. Armlet is indeed core on pretty much any STR hero looking to DPS. Its incredibly cheap, easy to build, includes great laning items (helm), and provides a ridiculous DPS increase for its cost. Particularly on a hero like N'aix who can basically ignore the degen in an extended team battle, and who benefits so much from cheap IAS, Armlet is good. Treads again, pretty standard on most DPSing heroes, and like I said, N'aix gets huge DPS boosts from IAS.
The game now is moving faster and faster. Even bad pubs generally don't take more than 30-40 minutes. You need to be a somewhat effective carry as early as possible, so spending all game farming your A/C, Heart, Buriza, etc. won't do your team any good in most cases. Deso gives you the biggest DPS increase for the cheapest price and makes N'aix a beast much earlier than many carries. Really, he can be a presence with just Armlet and Treads. Armlet->Treads->Deso is a good build order. The only time you should deviate is if the other team has mass disables and you find yourself unable to do your job with just rage. In this case, Armlet->Treads->BKB is fine. Otherwise, aim for a BKB after Deso if you seem to have trouble finishing some people off.
Cuirass/Heart are good, but are ultra luxury items that don't provide enough benefit fast enough if you get them before the others. In general, I'd say Cuirass would be better than Heart, however. With Rage and BKB, casters won't touch you before you 2-shot them, so the EHP from Heart over Cuirass is void, and Cuirass gives IAS which again, gives N'aix a great burst DPS increase. -armor is also good for high armor targets that reduce the effects of Feast as well.