[michaelsanford]

This may be a bit late but I'll try nonetheless.

If I get you properly, the AirPort base station is plugged into your router which provides DHCP addresses, right ? Well in that case, you don't want the AirPort to assign IP addresses, you want your router to do that so there aren't any conflicts anywhere. That's precisely what's happening when you enable DHCP on both routers : your wireless network gets 10.0.0.X and your linksys network gets 192.168.0.X (or something similar).

When you disable DHCP on your AirPort, but for some reason can't browse with wireless machines, do the wireless machines even get an IP address ? Did you have static DHCP mapping enabled ?

Also, if you are just using the AirPort to extend your wired network (and let your primary WAN router assign IPs) you don't need to forward ports with your AirPort, just the wired router.

PS PM me if you want, you can send me your AirPort configuration file and I'll take a look at it...