Wired sensor project to Wireless, Zigbee?

Hi, I want to change my project from wired to wireless and have been looking into Zigbee to do it but if there are any better alternatives I would jump to that. The range I am looking for well within what Zigbee is capable of. This is the basic layout and sensors for my wired project:

I would like to make my infrared sensors wireless but I don't know what would be the best hardware to get to accomplish the job, the cheaper the better since I may add more sensors in the future. I'm new completely new to going wireless and any advice on what hardware I should get would be appreciated, software and programming have never been a problem once I know what I am working with.
Thanks!

Hi, for lowest cost the Nordic Semiconductor nRF24L01+ series of modules can't be beat.

These are very capable radios but take a bit more software development than "Serial replacement" like Bluetooth or Zigbee.

See this to start on the http://ArduinoInfo.Info WIKI: http://arduino-info.wikispaces.com/Nrf24L01-2.4GHz-HowTo

DISCLAIMER: Mentioned stuff from my own shop... and many other suppliers