If you're looking for a 1 lens fits all there will always be compromises however there are a few lenses that get close to achieving the impossible. I don't think your budget will get you a decent 2 lens kit.
I've heard good things about the new Sigma 18-200 OS lens
First things first, it is NOT an L lens so won't be as fast, sharp or as well made. But for the money you're getting a lot. It's probably better than the kit lens on both IQ and build quality.
I'd also throw in a nifty fifty (50mm f1.8 MkII) as that will help with the low light needs and is worth having anyway considering they only cot £50!
 Try Ian Kerr (Kerso) for the 40D as he has them on his ebay store for £575. You can then claim £100 from Canon via their cashback leaving you over £500 for lenses
I've spent probably £2k with Ian in the last 12 months and have never had a problem.