Great advice above. Couple small things -

- Make sure you're not in an historic district or have some other
requirement (like a neighborhood covenant) that would make you get wood
windows.

- If your house is worth a lot of money, don't get cheap windows.
Personally, I would NEVER get wood windows again, why buy something
that requires maintenance. YMMV !

Here's a big-ass articel from Consumer Reports, it's free he

http://www.consumerreports.org/cro/h...view/index.htm

Windows
Upgrading to energy-efficient windows will likely improve your comfort
and your home's aesthetics, but it will take years to recoup the
initial outlay from energy savings

You'll probably want to install new replacement windows when you're
remodeling, when the old ones have deteriorated, or when you want
windows that are easier to wash and maintain.

Modern windows incorporate a frame made of all-vinyl or wood, the
latter often covered in vinyl or aluminum, with two panes of glass.

To cut energy use, those panes are separated with air or another gas
and sometimes specially coated. Improved comfort in the summer and
winter is the major benefit, and slightly reduced heating or cooling
costs will be an added bonus.


WHAT'S AVAILABLE

Window styles include double hung, sliding, hopper, awning, casement,
and bay. The major brands are American Craftsman, Andersen,
CertainTeed, Crestline, Marvin, Pella, Simonton (which also makes Sears
models), and Weather Shield.

Some brands are sold at home centers such as Home Depot, Lowe's, and
Menards. But most brands, including Sears, are typically purchased by
contractors through distributors. Some windows come in custom sizes;
others in stock sizes only. The materials that make a window frame can
affect energy efficiency, maintenance, and price.

Price range: $150 to more than $400 for 3x5-foot, double-hung, double
glazed windows.

Vinyl. These frames are easy to maintain, but they aren't usually
available in many colors. And they are sometimes difficult to match
with existing woodwork. Vinyl windows are often sold as a low-cost
choice. Many were lower-performing models in our tests.

Aluminum. As vinyl frames have become more popular, aluminum frames
have become less so. The biggest drawback is that they allow heat to
escape. That can make the area around the window chilly. In places with
cold winters, a simple aluminum frame can become cold enough to
condense moisture or frost on the inside, but where winters are mild,
aluminum can be a good choice for its durability. If you are set on
buying aluminum-framed windows, choose ones that have "thermally
broken" frames, with insulating material between interior and exterior
components.

Wood. For elegance, wood is difficult to beat, although it usually
costs more than vinyl and requires painting or staining and other
maintenance. To minimize maintenance where it's usually needed
most--the exterior side--many manufacturers cover, or clad, the wood in
vinyl or aluminum. Wood composite frames--some made from a mixture of
wood fibers and plastic resins--are supposed to combine the durability
of wood with the low upkeep of plastics.


Important features

Three types of glazing are commonly available: single, double, and
triple. A single pane of glass, or single-glazed, allows the highest
transfer of energy and offers little insulation against frigid winters
and searing summers. Double-glazed windows have two panes of glass. A
few manufacturers offer triple glazing.

The gas between the glass has a bearing on the quality of insulation.
Plain old air works fine and is standard for some brand lines. Argon
gas, which provides better thermal performance, is standard in other
brand lines; sometimes it is a step-up option. A few top-of-the-line
windows incorporate krypton gas, which provides incrementally better
insulation.

Double- and triple-glazed windows are sealed assemblies so they retain
any special gas between the panes and also keep out moisture, which can
condense between the panes. Should the seal fail, moisture, water
droplets, and fogging between the glass panes can occur.

Clear glass lets a relatively large amount of radiant energy (heat in
from the sun during the summer, heat out from your home during the
winter) to pass through. Low-E coatings (the "E" stands for emissivity,
or the ability of a surface to emit heat) enhance the insulation
quality of a window by making it reflect heat.

These coatings reduce some of the visible light that passes through the
glass and may give a tinted appearance. The view out at night may be
impeded somewhat. The coatings can be fine-tuned for different
climates--a southern or a northern window, for example.

Most new double-hung windows have tilting sashes, a very handy feature
that lets you pivot them inward for easier cleaning. With most, you
simply flip a lever or two to tilt the sash inward. But with some, you
must pull the sash out of the track.

Mullions are decorative vertical elements that separate panes of glass.
To help keep out water, some windows have a thin lip-a strip of wood or
vinyl about an inch high-that rises from the sill. You'll need to work
around it when installing a room air conditioner.


How to choose

Performance differences. Consumer Reports has found most windows do a
very good or excellent job at sealing out a fairly strong wind when the
outside thermometer registers 70° F. Only a handful do well at sealing
out a high wind when the outside temperature drops to zero. When it's
that cold, weather stripping and other components can stiffen or
shrink. Our tests have shown that aluminum frames are durable. But we
have found windows with frames made of vinyl- or aluminum-clad wood can
perform well, too.

Recommendations. If you're replacing windows, choose those that are
designed for your region's climate. Cooling costs predominate in
southern regions, so look for double glazing and a low-E coating. Give
first consideration to windows with a low solar-heat-gain coefficient.
The Department of Energy recommends that the number be 0.4 or lower.

Heating bills are of concern in northern regions. Give priority to
well-insulated, double-glazed windows that are draft-free. A low-E
coating isn't essential in places where summers aren't particularly
hot. In central regions, both heating and cooling are concerns. As in
southern regions, look for double glazing and a low-E coating. You'll
also want high insulating performance and a solar-heat-gain coefficient
of 0.55 or lower.