AVR-Chat

Range with Maxstream - I obtained 3/4 mile (could have been more).
Simple test conditions: XBeePro module with little 2 inch whip
antenna, held out of car window, talking to their USB module @15 ft
elevation. The USB module is another XBeePro with a slightly better
antenna. Pretty much line of sight. With better antennas, you'd do
more. With XBee instead of XBeePro, the range would be much reduced.

900MHz modules. As far as I know, only ZMD is making these for
802.15.4 and ZigBee is delayed. Few people want 900MHz ZigBee because
it's legal only in No. America. It is slower speed. A new version of
802.15.4 due in a few months promises to speed up the 900MHz version
of the standard.  In the EU, these modules are often tweaked to run on
868MHz. The 902-928 band in No. America has lots of channels whereas
the EU's channels are few. 

2.4GHz 802.15.4 works worldwide with the exception of two or so don't
care countries. Of course 868/900MHz propagates better than 2.5GHz,
but not enough better to offset the fact that there are numerous 60mW
(Pout) radios for 2.4GHz and I've never seen more than 1mW for 868/900.

Antenna gain is easy to get, if you can stand the size.