Just remember to setup one of the routers up in bridge mode.
If you run them both in router mode, you'll wind up with two totally separate subnets were devices can get to the Internet, but devices won't be able to talk to devices on the other subnet.
At my place, I gave each wireless network a slightly different name, but I've read posts on here where people use the same wireless network name on both routers. I went with the different names so I could force a 'n' device to join the 'b/g' network if I wanted to, and I could never get that to work when both routers used the same network name.
The whole thing is pretty easy to do, but might take a little bit of goofing around before you wind up with the configuration you like.
But like J&JPolangin mentioned, you can pick up a cheapo Linksys for your 'b/g' network. If you can't figure out how to put that guy in bridge mode, then run it as the router, and use the Extreme in bridge mode for your 'n' network.