There's another in-between option too. My boyfriend has a really nice Canon that's almost like a mini SLR (his is the Canon Powershot S2 1S which they don't make anymore because it's a few years old, but they still have similar, newer versions). You can swap out lenses, but they don't make as many as the EOS line. But you probably wouldn't NEED any more lenses- I have a Canon 20D with an upgraded lens (it has a slightly wider range than what they normally come with), and he can zoom WAY farther than I can (just using the optical, not the digital, zoom), and it has really nice macro capabilities too. I think he paid around $400 for it. I see that the newer ones also have RAW capturing abilities too, which is impressive.
If you do decide to go with a DSLR though, I would really highly recommend that you get a set of magnifying filters for your lens instead of getting a macro lens. I've had a set of Hoya filters (I think they're called "close-up" filters) for almost 10 years, and I paid $11 for them and they work SUPER well for macro photos. It's basically a set of magnifying glasses that you screw onto your lens.