Pix of the replacement air filter. The Sentra (Sunny in S'pore) uses part number 33-2031-2. Do not use any other part number. It's the same for both the QG16 and QG18. Even the SR20DE uses the same. Box label also states the manufacture date which in my case was 11/22/2006.
Pix of the K&N compared to stock paper element air filter
Compared to the Nismo and stock air filter the K&N is washable (recommended every 25-30K km or so). Installation was a zilch and took me a max of 15 minutes including cleaning my airbox cover. Should take less than 5 min if you just replace the filter. All you have to do is unlock the 4 metal clips surround the airbox lid, lift up the lid (be careful as the air intake piping and airflow sensor is connected to the lid), remove the stock filter, put the K&N filter in it's place, close the lid and lock it back. Make sure the K&N filter is properly seated before you close the cover.
Took the car for a drive and the difference was fairly minimal. Can feel that the engine is "looser" and revs a bit more willingly. I would say a max of 1-2hp at most. Not too bad for something so cheap. Still gonna try and compare with the Nismo air filter though.
Jan 22nd 2007 Update: Let me take back my words on the "minimal" statement I made earlier. Took the car for a drive today to Batu Ferringhi and the difference was quite a bit. Noticeably better pull from the midrange to the highs. Engine feels less strangulated. Guess the ECU needed some time to adapt to the better airflow.