There is a spreadsheet for calculating WiFi range here:
http://www.cisco.com/warp/public/cc/...it/obrc_in.xls
and a write-up of an experiment over 14km here:
http://www.frars.org.uk/cgi-bin/render.pl?pageid=1066
2km with proper line of site (see diagram in spreadsheet) should be possible with no probs.
Lots of bits and pieces available here:
http://www.solwise.co.uk/