R-tree, order by, limit

Hello,

I am implementing a map application. There are towns with altitude,
longitude and population.
One of the tasks is to be able to query N biggest (by population)
towns within a rectangle.

Something like (maybe the syntax in not quite right, but the idea is obvious):
SELECT * FROM towns where alt1 <= alt <= alt2 AND long1 <= long <=
long2 ORDER BY population LIMIT 10;

If I create an R-tree index on coordinates (alt, long) this will speed
up the query significantly. But it is still far from optimal: Despite
we need only 10 biggest towns, all towns in the rectangle specified
will be examined.

What if we include population into R-tree index? This index will
handle a 3D space with coordinates (alt, long, population).
Will this 3D index perform better than that 2D index?

In fact, I lack some details on how Postges handles ORDER_BY and LIMIT
inside R-tree indexes.
Extensive answers and links are appreciated.

Thanks.
Anton.