How much benefit you will see, if any, depends almost entirely on your overall network configuration and whether or not the other devices on your network are capable of operating on 802.11n. If the other devices on your network are 802.11n capable then you may well see some improvements by setting your network to 802.11n ONLY. If not and you have to use 802.11n with b/g compatibility mode then only the 802.11n devices will benefit.
One of the biggest lifts my network has realized by going to 802.11n ONLY resulted from shifting from the 2.4 GHz to 5 GHz band. There is a lot less interference from neighbors networks and normal household devices in the 5GHz band than in the 2.4 GHz band, which results in noticeably better network throughput and network stability.
As for AirTunes, if it was working well on 802.11b/g then it is highly doubtful you will notice any improvement by going to 802.11n.
