Camera bag frustrations. Please help.

I'm looking for a camera bag that is just large enough to handle what I have, but no larger. Here's what it needs to carry.
XS with 70-300IS USM attached (and stored if another lens is attached))
Tamron 28-75 2.8
Canon 18-55IS
I require just the most minimum storage for a couple batteries, and mem. cards.

I currently have a Lowepro Nova 4, which handles the requirements, but is larger than I'd like it to be. There must be something out there that can handle my needs, and is smaller. Ideas?
Needs to be a shoulder bag for quick access on the fly. No backpacks.
Thanks for helping.