I am trying to decide between the eos xsi and the 50d. I need lots of versatility while traveling and a good camera to take action shots. which one?

Both the XSi and 50D are very good cameras that will take sharp photos with high resolution for printing large and/or to allow for some cropping while maintaining decent resolution. Both have the EF-S lens mount and 1.6x crop factor. There are many similarities but also many differences which set these two cameras apart in use and consequently in price.

The 50D will be a better choice for your stated goals and here is why.

Goal #1: "I need lots of versatility while traveling"
I assume when you are traveling you might be gone from home for more than a few hours, you might want to take photos at night and inside dark buildings.

*50D high-ISO performance is better than the XSi and will allow you to attempt shots in darker places, therefore it is more versatile.
*50D has a larger viewfinder, its easier to see what you are shooting.
*The new LCD on the 50D is MUCH better for evaluating the photo quality.
*50D has lens autofocus micro-adjust, it can compensate for minor lens calibration issues and give sharper photos and also potentially reduce the amount of service trips your lenses and/or camera need to take.

Goal #2: "to take action shots"
Focusing speed and maximum frames per second are both much faster/better in the 50D. The Rebel series is built for low cost, not for speed. The 50D takes two photos for every one the XSi takes (6.3 frames per second vs 3), chances of getting those "killer shots" and having it actually in focus are so much better with the 50D.

For the Lens:
The new Canon 18-200 is a very good all purpose zoom, great for a single vacation lens and as a casual carry around.
For indoors or night sports you will find it cannot keep up with the action without using a flash. For subjects up to 25 feet away the 430EX should do fine, more than 25 feet you probably need a powerful one like the 580EX.

For times when a flash cannot be used you will need a "faster" lens to freeze action, meaning one with a maximum aperture of f2.8 or a smaller number. Unfortunately the f2.8 zooms are big, heavy, and expensive. Prime lenses are smaller, faster and cheaper, but you then need to find the right one or two lenses that works best for the sport you are shooting. You can get extensive information for choosing lenses at my favorite website, fredmiranda.com.

Does this help? More questions?

Happy shopping!