Well, Santa (and Grandma) have been good to me this year, and have given me the chance to upgrade to a new lens :)

I have about $500, and I'm looking at these lenses so far. They will mostly be used for shooting sports, with some landscape stuff:

I am looking for something around the 12-17 starting zoom, and the end zoom doesn't really matter to me, but I'm looking around the 30-50mm mark.

A couple examples:
http://www.bhphotovideo.com/bnh/controller/home?O=8433&A=details&Q=&sku=207357&is=USA&addedTroughType=categoryNavigation

http://www.bhphotovideo.com/bnh/controller/home?O=8433&A=details&Q=&sku=317364&is=REG&addedTroughType=categoryNavigation

http://www.adorama.com/SG1530NKAF.html

Scott-
Here's a page with a table comparing the three lenses you're interested in:

Compare Nikon 18-35mm, Sigma 17-35mm EX DG, and Sigma 15-30mm EX DG lenses >> (http://www.photographyreview.com/ProductComparecrx.aspx?pid=20724256-28802082-20723807&befid=96323)

There are no reviews yet for the Sigma 17-35mm EX DG, although there are a lot for the non-digital version of the lens, which should be pretty comparable: http://www.photographyreview.com/sf-1/befid-96323/pid-20724506/productreviewscrx.aspx?dnatrs=zoom_lens-less_than_25-less_than_50_mm

You should also take a look at the Tamron and Tokina options. They both also make excellent lenses, if you buy the good stuff.

Thats weird- I edited my post and it didn't show up?!

After looking at the reviews here, and Ken Rockwell did a wide angle shoot out, I am really leaning towards the Tokina 12-24. Its right in my price range, and some of the images I've seen with it are fantastic.