If its as a walkabout then a Domke F3(XB) will do all that and a lot more, and is as discrete as they come (once you remove the Domke labels from the pockets), just looks like a funky canvas shoulder bag to the untrained eye (available in various styles/finishes too) ...
I love mine - they are expensive but well worth it IMO ... I got mine at a reduced price off Amazon about 18 months back (less than 100 quid I think), but it has been used tonnes and still has hardy any signs of wear.
I regularly carry either a gripped 50D or 5D II with a 24-70 L or 24-105 L attached and still have a lot of room for a choice of 2 other lenses inside the main compartment (the 70-200 f2.8 or 100-400 L will also fit in there if you remove the lens hood). The outside pockets can carry blowers and batteries and all that kind of stuff, and you can fit a 580 EXII in its case in one of the oustside pockets (bit of a squish if its on the same side as the 70-200 though). Then there are thin front and rear flap pockets for note pads and stuff. Inside the main flap/cover is a mesh pocket for CF cards and stuff.
This bag is like a tardis due to the lack of padding ... don't get me wrong, everything is well protected enough for normal walkabout use, its just not full of padding like Crumplers are, and doesn't broadcast the fact that you are carrying expensive camera equipment like lowepro or crumplers do.
You might want to think of getting the extra postman's style shoulder pad if you plan to use it a lot ... or save a few quid by nicking the shoulder pad off a crumpler bag (and remove the badge) like I did 
EDIT: one thing ... it will hold off some water/rain, but I find the top flap doesn't cover the main compartment quite as well as I would like - remove one star for this, but as I live in Portugal, its not that big a deal for me most of the year
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