You have some conflicting requirements here -- something that can carry a bulky camera and a couple of lenses, but that you can get at quickly, so you can set it up on a tripod. The tripod means that you're not going to be all that fast, no matter what you do.
I once had a film Mamiya RZ that I would sometimes use for landscape shots, and tried a number of bags with. Almost any shoulder bag is going to be awkward for MF equipment, and tiring to carry. Best bet would be a backpack of some kind (as Kevin suggests) and there are several good brands -- look at Think Tank first, they cover almost everything. Kata has (or had) a bag that had two compartments, and includes do-it-yourself dividers -- the bottom one would be good for the camera, the top would carry a couple of large lenses, and Kata bags tend to be well-padded (some would say over-padded) and water-resistant, with tie-ons for a tripod. But no backpack would be really quick to use, like street-shooter gear, where you can shoot in a matter of seconds. The right pack would only be relatively fast, given the equipment you want to use. Also, be prepared to modify the interior of the pack -- don't think you have to accept what you see in the store. Look for a pack with velcroed, pull-out dividers, so you can re-divide the space. Bottom line is, go to a big camera store and look at Think Tank and Kata.
I'd also suggest that you forget about the tripod. Try hand-holding for a while, you may find that's good enough. Tripods are a pain in the ass, and if you're out walking the street, as opposed to streetwalking or doing rural landscapes, you may find that a lot of other people who share the street with you don't want to walk around a tripod, or worse, may run into it, and knock your camera over. There's nothing quite like the sound of a Leica S2 nose-diving into a concrete curb. In some places (like New York) it's against the city ordinances to set up a tripod without a photography permit, because it snarls up foot traffic.