I currently have my home wireless network secured by use of a MAC filter. Is it necessary to use a WEP key to secure the network or will the filter work just as good?
It is good that you are using MAC filter. This is one way of protecting your wireless network but adding a WEP key will give you one more additional layer of security for your network. If you just use a MAC filter - it's possible a hacker can use MAC spoofing to gain access to the WLAN.
An attacker can disrupt your WLAN network using some tools to change the MAC address of the access point. Almost wireless cards will allow you to change MAC address to a different manufacturer value.
Best security practice: Always use at least two levels of security. I would recommend you to use WEP Key too.