Heres what I would suggest you consider:
1. Bridge the DSL Modem and you can still use it as a modem for your ISP connection.
2. Get an external DIR series router. Any one would do however considier your current and future needs. Will you be using 5Ghz in the near future? Also consider that if your area has many other routers near by, 2.4Ghz could be affected by them and thus 5ghz would be a good option to have in addition to 2.4Ghz. However 5Ghz is also affected by building materials and is mainly used in Line of Sight. However it does work with good with in about 10-15 feet with one wood material wall, ceiling or floor separating the host router from the client. Any farther then that, signal will degrade.
I recommend reviewing the DIR-8 series routers, 825, 827, 835, or 857 for 2.4 and 5Ghz WiFi, or if you don't need 5Ghz, the DIR-645, 655 or 657 routers. DLink is comming out with a new DGL-5500 Gaming router which is a follow on to the DGL-4500 which was a great router for gaming. I have 3 of them.
I can't wait to get my hands on the 5500. Not sure where it will be sold as far as regions yet. So you might want to hold out and see.
Check it out:
For those of you who dreamt of the next gen DGL series router, Prepare yourselves!!!If you can get into a 5500, then you may want to upgrade the xbox WiFi from 2.4 to 5Ghz using this:
DAP-1533Or if you really wanna go over the top if you get the 5500, and upgrade the connection between the 5500 router and the xbox with using AC WiF, you could get this for the xbox upgrade:
DIR-865L.
This can be bridged wirelessly, not wired and will provide an Wireless AC bridged connection to the DGL-5500. I have the 865L, nice router. Will be a nice WiFi bridge for the DGL-5500 and xbox. You would use the LAN cable from the xbox and connect it to the back of the 856L in bridged mode.
Again, check to see what is available in your region.
These are some suggestions to help you get a better connection between the xbox and your other devices.
3. Depending on what you get and how it's all configured, you shouldn't need an external network switch, however I would recommend one anyways as it is very handy to have and does help if you have other wired devices on the network.
Let us know if you have any more questions or need help.
It's possible that maybe someone can take a look at the modem settings to see if there is any thing we can do remotely to configure the modem so you can get media center working better. Might be worth a try. If not, You'll have some thinking to do. Let us know what you decide.
I hope we can be of help.