Upgrade lens for XSi

I currently have a Rebel XSi and I would like to invest in better lens(es). Right now, my budget is in the $500-800 range, although I could go slightly higher if there is a great lens just out of reach.

I mainly like to take travel pictures when I go to Europe and I would like to do more of that in the US as well. I also like to take some night shots of cities as well as taking portrait and group photos of people usually indoor and sometimes in lower light conditions.

I am debating whether I should get another zoom lens or get two high quality prime lenses. I looked at the Canon 20mm f/2.8 in a store and the store person thought it would be a perfect prime along with the Canon 50mm f/1.4. I read some reviews online about the 20mm, and some people really like it; others thought it was terrible; and some thought it wasn't good for a cropped camera like mine. I'm tired of reading contradictory reviews, so I thought I would ask people who use these lenses.

My other thought is getting the Canon 17-40 f/4 L. Perhaps this wouldn't be as good for the low light situations, but I could get this as a walkaround lens and then get one or two primes in the future. It seems that people really like the 50mm f/1.4, so I probably will get that at some point.

So here are my questions:

1. Should I get one zoom (perhaps the 17-40L and then get prime lenses down the road)?

2. Should I get two prime lenses, such as the 50mm f/1.4, and something in the 20-30mm range instead of a zoom lens to cover that range? If so, what would people recommend in the 20-30mm range?

3. Or based on my description above, should I go in a completely different direction?

Thanks for the advice!