The 24-105 f/4 IS is probably THE all-around lens, but 24 is a little long on a 20D, plus it's over a grand. The 17-40 f/4 L is the lens I used most in Napa/Sonoma. It's a little short for portraits, but if you're also bringing one of the good concert lenses (50/1.8 or 85/1.8 ), you'll have that covered. The 17-40 is over $500, though.
The 17-85 f/4-5.6 is probably most of what you're looking for and is right around 500 bucks. It's a slower lens and the glass isn't as good as either of the two I mentioned above, but it will do the job.
If you're comfortable with switching lenses, try the three primes 24/2.8, 50/1.8, 85/1.8. This really isn't a beginner's setup, and you don't give any indication of your skill level. This definitely isn't a "default lens" recommendation.
I hope that helped. For that price, you will have to make some kind of sacrifice, either in flexibility (slow zoom) or in ease-of-use (primes). I love my 17-40. The other nice thing is that it's L glass, so you can resell it at a good price.