I use the Canon EFS 17-85 IS.

Very pleased with it. I bought it as part of a Canon kit though, not sure I would be quite so willing to spend the full price. Is the IS necessary to you it seems to add a lot to the price of the lens.

Never heard a bad word said about the standard 50mm lens, I have seen some superb portraits done with that.  The crop factor on your Rebel XT makes it into what is considered the ideal portrait length. Personally I would go with that and use the money saved on something else if I were doing portraits.

I have a Sigma DX lens too the quality is outstanding - several people have compared it to their Canon lenses and agree. The only comment I have heard on the negative front is the build quality is not as robust as Canon L lenses with regards to throwing it around and being out in all the weathers. But they are certainly well built and will stand up to many years of amateur use, image quality is comparible.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html