How much speed improvement 802.11n (5 GHz) will give you over 802.11g (2.4 GHz) depends on the nature of your network. If you setup an n Airport Express, or as I did a Time Capsule, so that all of its wireless clients are n devices the networks speed improvement over having both g and n devices served by the n router should be considerable.
To get the maximum benefit out of n speeds, I have setup my Time Capsule so that its only wireless clients are my MB Pro and Apple TV, both n devices. My PowerBook G4 and TiVo Series 3, both g devices are still served by my old g wireless router, a Belkin pre-N, set to g only. The g router is connected to the Internet via the Time Capsule, so the MB Pro can see both networks and I can use it to select which to use. The Airport Utility took care of the configuration fairly easily.
The short answer to your problem seems to me to be that unless you plan to do backups wirelessly on a dedicated 5 GHz network that has only n wireless clients, the old g AE would probably serve about as well as the new n one.