The IRIS plugs are the BEST zigbee repeaters I've seen so far...if you can still find them. model # 3210-L
Also, look at this Xbee thread:
IF you don't mind going this route...these really end up being amazing zigbee repeaters. However...using this software and mapping my network showed me how well the IRIS sockets were. I'd say you really don't need the XBEEs if you have a handful (depending on your house) of the IRIS plugs around. That being said I've found it very helpful to keep an XBEE attached to my home desktop just for mapping my network and checking things out occasionally. Plus they are really cheap.
My Zigbee network is by far much faster and more reliable than my zwave network.