I believe zoom refers to the amount of difference between wide angle and maximum telephoto. It's a deceptive word -- if you had a lens that had 20x zoom but its wide angle was 12mm, you'd only be looking at a 240mm lens on the long end. On the other hand, a 100-400L has a 4x zoom, but is 400mm at the long end.
Zoom is versatility, but focal length (mm) is magnification. The more zoom, USUALLY the more tradeoffs (IE maximum aperature, image quality, distortion, abberation, WEIGHT, etc).
Money not an option? Get a 400 F2
.8 and go from there. Sharp and fast, but expencive... or you could get a 500 F4. E-mail me your address and the hours you're at work, as well.