hey- my advice is to get an inconspicious-looking (=cheap) messenger-style bag, and a couple of Domke inserts. Add a little foam padding to the Domke inserts, as they are not only padded on the sites, strangely enough. This is inexpensive, flexible, and does not attract too much attention.
keep in mind to get a messenger bag with a flap that you can make friends with - some of them are just plain cumbersome, and you'll - presumably- want quick, unhindered access to your stuff.
Also - another reason for not buying a dedicated camera bag is that these usually leave room for stuff like a rain jacket, drink bottle, books, and all the other paraphernalia that you usually carry around with you.