Watchnerd, welcome to the discussions!
"I get a lot of interference and audio streaming drop-outs in 802.11g mode." If you are getting interference and drop outs in "g", your performance may likely worsen in "n" mode.
The reason that your Express is sometimes dropping down to "g" now indicates that your broadcast signal is either not strong enough or already has significant interference issues.
Download iStumbler, www.istumbler.net, to get a look at your wireless network to check the relative strength of your signal and noise on your network. iStumbler will also display other networks in your proximity so you can see some of your competition.
If you find another network that is operating on the same or close adjacent channel as yours, change your channel using AirPort Utility to another channel that is as far away as possible from other channels in use.
Unfortunately, iStumbler will not display networks with hidden names. Most newer routers provide this feature, so it is possible and probably likely that there may be other networks interfering with your network that you cannot identify.
Cordless phones in the house? They are a primary cause of interference. Security cameras nearby?
You can set your Express up for 802.11n only using AirPort Utility. Is that what you are referring to when you say "force" it to use 802.11n?
You didn't provide any details on your main router. Is it an Apple as well? If so, you may be able to make use of the "extend a wireless network" feature in the Express. This almost always provides better performance than "join a wireless network".