Anne-Louise,
I agree with Phil, the 300mm will give you more flexibility. Many places do a deal on the 400D + 18-55 + 70-300 deal (like Photocontinental) for about $300 over the price of the body itself. Neither lenses get great reviews but nonetheless this represents exceptional value
.
I definitely know that the 18-55 kit lens can do excellent work. In fact some people are doing amazing work with that lens, see for example:
http://www.pbase.com/zylen/image/52939326
If you want to know what the 75-300 is capable of have a look here:
http://www.pbase.com/cameras/canon/ef_75300_4iii
This setup should suit your needs while you decide whether you want spend money on better lenses which will give you benefits like extra sharpness, faster focal ratios, Image Stabilization, better construction, etc.
Good luck on your decision.
Terry