I've heard good things about the 400D.
I think some of the butterflies are seasonal. But not many. You won't run out of subjects for your photography. In the summer it's really hot in there.
All my butterfly shots are with the Canon EF 100mm f/2.8 macro lens. My favourite lens by far!
Let me know when you get the 400D. If i've got the time then we could meet up at the Papiliorama and i could let you use my macro lens and my extention tubes and you can decide if you need one or the other (or both) yourself.