But does it have the chip?
Even if it did, only B/G are enabled on all models, so whatever speed increase your seeing is due to some other factor.
Which leads me onto my next point, even if it did have N, that is regarding the wireless speed on the
local network, ie. the speed devices can communicate with each other in your house.
You could have Wireless Z (doesn't actually exist, I made it up), which runs at 1 billion billion billion Mb/s, but if your Internet provider is giving you 10 Mb/s broadband (as an example), thats the maximum speed you can access the internet at.
Faster wireless routers allow things like, faster transfer of files from another PC on your network, ability to stream HD video from another PC on your network, and of course better wireless coverage.