Need help : Arduino with Zigbee/Xbee , GSM and Wifi capabilities

Hello All,

I'm doing project where I need Arduino to have Zigbee/Xbee radio, GSM and Wifi capabilities. Which board should I go for and how do I integrate all of these together? I can go for mega and purchase wifi shield which uses SPI and GSM shield which uses UART. WHat about Zigbee? How do I connect all the shields to Mega?

Any help will be really appreciated!