New furnace?

I've been thinking about replacing our furnace & air conditioning unit next year, in 2010.  Although ours aren't that old (they are currently 11 years old), they are low-efficiency and result in much higher gas & electric bills than we should have for such a small house. 

The federal energy efficiency tax credits will reimburse us 30% of the cost, up to $1500, for a high-efficiency replacement furnace & AC unit.  I'm not sure how much a new unit will cost, but I would imagine it would be in the $5000 range.