Can I turn off the wifi in the 2Wire (Model i38HG)
Yes, if you are able to access the configuratiion settings for the 2-Wire router. Typically, they are found by typing a default IP address into your browser.
and hook up via ethernet an Airport Extreme to the 2Wire and use the Airport Extreme as the wireless router?
The AirPort Extreme can be configured as a wireless access point to provide a wireless signal for devices. If you have a newer dual-band AirPort Extreme, the Guest Network feature cannot be enabled in this type of setup.
To hook this up, you would run an Ethernet connection from one of the LAN <-> ports on the 2-Wire router to the WAN port (circle of dots icon) on the Airport Extreme. Then, confgure the AirPort Extreme to "create a wireless network" in a Bridge Mode configuration.
I would like to do this because I also want to be able to use an Airport Express to extend the wifi signal to a far area of the house where the signal is weak.
Both the AirPort Extreme and AirPort Express will need to be "n" versions to support the "Extend a wireless network" configuration. If that's the case, and The AirPort Express is located where it can receive a good signal to extend it, then this will work.
Post back if you need more configuration details for the AirPort Extreme and AirPort Express.