Lots of confusion here...
Hey, all!
First, yes, connecting a b/g device on an n network will slow the network somewhat. But only when that device is active - if your 360 is off & not downloading updates or anything, the network will be running full speed ahead.
It does not, however, slow completely to the speed of that slowest device, even when that device is active. So, your macbook will get full N speed when the Xbox is inactive, and get fairly close to N speed even when the 360 is active.
If you're very worried, you can easily run both, with different SSID's, as others have suggested. Then you divide the network into an N network, and a B/G network. Connect the two base stations via ethernet, and you're good to go. So long as only N devices connect to the N base station, and all others connect through the B/G station, the speeds will be preserved.
Ethernet devices will not interfere with the speed of the local network at all. The slowdown when an older B/G device is connected is only a factor of the radio chipsets being used for multiple standards. Since ethernet will not be using the radio at all, all your wired devices will get full speed, even if you've got an ancient B device on your wireless.
Know also that unless you've got a 100Mbit Internet connection, you won't see a difference for most things. The only exception would be local file transfers (re: backups to TC), and possibly streaming media on your local network. Anything online is going to be easily capped by the bandwidth you get from your provider; most US Internet connections have yet to beat even 802.11b.
And to the guy with FIOS - I hate you.
Hopefully that helps a bit.
-rand()