One thing to throw out there, unless you're watching netflix or downloading content to the 360, if you're just using it for game connectivity to xbox live, the amount of data sent is negligible. Its more about response time/latency than traffic volume.
For my 360 I went wireless but to solve the problems with the expensive slow xbox wireless adapter, I picked up a super cheap 5GHz wireless N router that was compatible with dd-wrt, installed that and set it into 'client' mode, then plugged my 360, roku player, ps3 and some other stuff into the back of it. That gave me a 300Mb/s connection back to my main router and multiplexes all those devices through it. I only use one at a time, so I get full bandwidth and my throughput and latency/response time are great.
Definitely some heavy technical stuff to install and configure the new firmware, but if you're patient and follow the instructions for the dd-wrt install and the client mode configuration, it only takes an hour or so.
This is the router I used:
Newegg.com - Recertified: NETGEAR WNDR3300-100NAR IEEE 802.11a/b/g, IEEE 802.11n Draft 2.0 RangeMax Dual Band Wireless-N Router
Here are the dd-wrt installation instructions:
Netgear WNDR3300 - DD-WRT Wiki
This is a good detailed technical thread on installing dd-wrt and the technical issues that may arise:
DD-WRT Forum :: View topic - Peacock Thread-FAQ: EVERYTHING you NEED to know! Really!!
Here is the detail on configuring the 'client' mode. Once this is done, this router looks like a wireless N client adapter with a four port switch on it instead of a single connection:
Client Mode - DD-WRT Wiki
This particular router is excellent for the purpose. There are also many cheap 'refurbished' units available because netgear sent out a whole production run with bad power transformers, and because there was some confusion about how it worked when both the 5 and 2.4GHz radios are in use at the same time. You could only use wireless G on the 2.4 if you used N on the 5. That ticked off some people who thought they could run N on both radios concurrently and they returned it. You only need one of the two radios to use it as a wireless client, so thats irrelevant.
One note is that the default client mode setup puts everything hung off the client mode router on its own subnet, ie your main router will be 192.168.1.1 and the client mode router will be 192.168.2.1. MRV and Directv2pc will not work between units on two separate subnets, so this works great for game gear, roku boxes and pc's but not for networking remote directv stuff.
You CAN make it work as a single subnet by disabling dhcp on the client router and doing a bunch of fiddling with it. I made it work once but it took me hours of fiddling.
This is also not a bad little cheap router for primary router usage if you understand its limitations and it suits your needs.