Aircrack will work for discovering networks, passively sniffing traffic and cracking the keys, however I highly doubt packet injection will work.
It seems to me it is not supported by the wlan drivers.

So this leads to:
- WEP will be somewhat easy to crack, as you only need enough traffic over time
- WPA/WPA2 will need packet injection to function or a long time of discovery (you need to capture the 4-way handshake, that's spread across channels. It will only happen if a client is disconnected and reconnects or connects for the first time. That's the thing you archive through injecting disconnection packets).

Regarding WEP it would be theoretically more efficient to use your N900 as a capturing node and decrypt the packets on your desktop computer. Afaik there is the possibility to use GPU acceleration, which will boost the whole thing a lot.

Keep in mind: Only mess with the network you own