Should I buy a one stage furnaces or a two stage furnaces for our 1350
sqf house? ($2.50)

The benefit of a two stage furnace comes from the fact that it can run
longer while putting out fewer BTU's to keep the house a more constant
temperature.  While it runs for a longer period of time it uses about
the same or a little less energy than a single stage furnace.

The monetary benefits of a two stage furnace are minimal at best. 
There is a higher initial cost along with the possibility of higher
repair costs, with little energy saving ability.

http://ths.gardenweb.com/forums/load/hvac/msg1021450611847.html

http://www.warrenheating.com/prod01.htm

Bottom line is that it is a comfort issue.  If money is not the option
a two-stage, variable speed furnace will give you the best comfort. 
If you want minimal upfront cost, go with a single stage furnace.  If
you don't mind a much larger upfront cost, but want long term saving,
go with geo-thermal.  There are obviously variable to consider, but
the 10 year savings of a geo-thermal system can be substantial.

http://www.eere.energy.gov/RE/geothermal.html

Thanks!