Actually, financially speaking, upscale vinyl window replacement had a higher return on investment last year (2008) than kitchen remodels! If you can afford it, you'll get nearly 80% of your money back, not to mention the savings you'll gain in your energy bills AND the Obama tax credit ($1500) this year!
Kerry, as someone who has been in the window industry since I was 16, this is what I would suggest you look for:
-Double-paned: Triple paned windows are much, much more expensive and do not save you much more in energy unless you live in a climate with excessive winter chill (such as in the mountains). They also can create a tremendous stress on the framing of your house because the additional glass is very heavy.
-High quality glass, coated in LoE3. The glass makes all the difference. It should have an argon fill (krypton does the exact same work, but is again much more expensive). The coating is imperitive; LoE3 is an advanced technology which reflects sun rays away from your house in the summer and keeps the heat inside in the winter. The LoE3 is also what makes windows qualify for the Obama tax credit this year.
-A good spacer. I personally recommend a superspacer, although many window companies don't offer this. A broken window spacer will leak out all of the argon gas within just a couple years, lowering the energy-efficiency of your window significantly. Beware of metal spacers (metal is a conductor) and also glue-based spacers (melt in the heat). The superspacer is a special composite developed by NASA which will hold your gas in for a lifetime.
Also, always always look into the window company you choose prior to doing business with them. I highly suggest checking out the BBB website as well as Angie's List before you sign a contract.
If you live in the Pac NW, Northwest Home Improvement Co. specializes in high quality window replacement. Hope this helps you, and happy hunting!