Since you want a microphone port, I assume you're also wanting to take video.
If you don't need manual exposure and 1080p video modes, the Canon 1100d seems like a good place to start. Check out the review at dpreview.com. I think it has a microphone port. (edit: the 1100d does NOT have a mic port, sorry).
Alternatively, with 1080p and manual exposure in video modes, look at the 550d or 600d if you can stretch a little bit more. You can get a 550d with kit lens on your budget approx.
However, you'll also need to plan for any other lenses (what do you want to photograph?), a tripod, filters etc. So the 1100d price range would leave you with some money for those things.
I don't know a lot about Nikon cameras, but I think the d3100 is their entry level dslr and i've heard it's very capable but has no autofocus built in and no 1080p or manual exposure in video.