Experts, weigh-in on my home network config

[CashGap]

The picture worth a thousand words is at the bottom of this post.

Looking for Airport configuration advice as I modify my home network. Everything is at latest firmware versions.

1. Broadband Source - Comcast Cable.
2. Broadband Modem - Motorola SB5101, configured and working as router. ISP doesn't allow it to bridge.
3. Two Airport Extreme's... early "n" MA073LL/A and brand new MB763LL/A.
4. One Express, need it for Airtunes.
5. HP printer with Ethernet.
6. USB hard drive.
7. Six g clients, three n clients as shown.
8. No coverage constraints (small house, thin walls).
9. One Airport Extreme, the HD, the printer and the Express must be in a certain location.
10. Incoming cable, cable modem, one Airport Extreme must be 25' away from #9.
11. I prefer to run no cabling, so the two Extreme's should use WDS.
12. Prefer the "n" clients have maximum performance.
13. Not really concerned about guest clients.

So, should the new Extreme be by the cable modem, or over by the HD and printer? What's the optimal setup, frequencies, etc.? I basically need an n only and a g only network, correct?

Looking for input, y'all know these products, I'd eventually muddle my way to a good configuration but would like to eliminate some trial and error.

All assistance appreciated!
Drawing

 

[SqB]

put the dual bander on the modem, use single bander extreme to extend 5ghz network, use express to expand g network.

that's how mine is set up now and it works great.

 

[CashGap]

Thanks!