trying to set up multiple Airports with a BT Home Hub

[t0mat0]

Hello, firstly, I know that there has been previous threads on Home Hub e.g. . I don't want to bump an old thread, as I've got a different set of problems/queries for a friend of mine. I'm setting up his network etc, and need some advice! If a mod thinks it's more relevant to post this on an old post, please get in touch as I don't want to cause hassle.

Basicallya UK set up -
1x BT Broadband with a new recent Home Hub
1x Intel MacBook
2x stereos I want to link in via 2x Airport Express (I have the relevant cables).
1x Airport Extreme
1x USB external harddrive formatted with a mac to create 2 partitions - 1 with a iTunes library, Time Machine partition for the other.

1) I'm looking to set up an 801.11n rather than b or g network,
for the Mac to be able to interact with the 2 external speaker systems through the 2 Airport Expresses (AE), so that when I add other devices e.g. a PC/Mac with 801.11n support, I can keep the n speeds.
2) I'm looking to be able to wirelessly access the iTunes library on the external drive. I'm thinking it's easiest to connect the drive to the back of the Airport Extreme via a USB Hub which can then also have a printer attached if need be.
3) I'd be attaching the Airport Extreme to the router via ethernet.

So I'm trying to work out how to get the bits to play nice. I've got the Airport utility - i'm just trying to work out which setting in the set by step set up I need to use. I preferrably want to be able to access internet through the same 801.11n network rather than have to sign out of the music and external drive 801.11n network shift to the access the BT Homehub and the internet.

I'd be wanting to have WPA or whatever best security was available due to the iTunes partition being linked to the wireless network. I know the Home Hub is set on default as WEP, so I guess that is another parameter to change for the Airports and the homehub router.
Any thoughts?
I don't have access to the mac this weekend, but should be able to get some screenshots of the choices the Airport configuration app offers. I've had a quick look at the interface of the HomeHub itself, but imagine there are some setting i need to access in the "advanced" section on the 192.x.... bthomehub page...

Potentially the colleague might want to try and add an Apple TV/Time Capsule, (for wireless backup and also to have a decent way to get movies on a HDTV), but I thought i'd try and help set him up with the basics first!

I'd be more than happy to try and help other people out with BT Home hub set ups once I can get my head around it - it's definitely worthy of a picture by picture walkthrough, for those less used to accessing the innards of their router i'd imagine.