I work wireless for a living.
Don't use WiFi if there's any possible way to use cat5/wired.
Wireless is about mobility.

QoS for VoIP in 802.11 WiFi is a real problem. Esp. if the client device is battery powered and using 802.11's power saving mode (sleep between beacons).

In the 2.4GHz band, the proper channel numbers are 1, 6 and 11. The trick is to find which one is least-used when YOU are using that channel. Common PC software shows you only the existence of SSIDs, not how busy these are.

Going to 5.8GHz with 802.11a devices, or for $$$$, 802.11n in 5.8GHz (dual band routers, etc.) gets you away from the crowded 2.4GHz band.