Yep, I have to agree with John and H, the 24-105 F4 Lis a very good lens.
It's wide enough to be useable on a crop body and the 105mm end gives good reach.
The 24-105's only vice is moderate CA at the wide end and a touch of barrel distortion, both easily corrected in Canon's DPP software, overall it's very sharp and has excellent contrast.
I had the 24-105 at the same time as the Sigma 17-70, after buying the 24-105 I never bothered using the Siggy, the difference was that great.
Then there's the bonus of Image Stabilisation....
From memory you already have a 17-40 F4L that would compliment the 24-105 nicely.
I know it's out of your budget range, but a good second hand 24-105 just sold for $900 on
http://www.ausphotography.net.au classifieds (you need to be a member to access the classifieds). If you can stretch the budget and are prepared to wait there will be more for sale.