Lowe Pro Nova 2, last year's version if you can still find it.

Room for camera with zoom+hood attached, one extra zoom lens (or two primes), external flash (or another lens, up to 70-200), and room in the pockets/mesh for everything else (cellphone, GPSr, cache goodies, ...). This was my handbag for last summer in Europe -- it went with me everywhere, and worked wonderfully.

Cheers