I picked up a Montane Dragon 20l the other day and am happy to report that I can fit a 2-3 day kit in it w/o too much problem. I haven't picked up a waterproof bivy yet, but loaded the pack w/ my Cricket tarp and superlight bivy- once I get the bivy (having a hard time shelling 400 bones for a bivy!) it'll save a little more room and 5-6 oz.
The Dragon is laid out nicely, the front harness carries the two water bottles (which are Steripen friendly), has a zipped pouch that will be just right for a map & compass, steripen and my camera, two additional stretch pockets that I can load up w/ blocks, bars and electrolytes. Also has a pretty slick setup for carry trekking poles. The rear of the pack has a nice bungee cord setup for rain jacket and windshirt. Beneath it is a pretty large zippered pouch- it's not waterproof, so I have small cuben dry bag that will hold my first aid kit, headlamp, toiletries, tp and various other little sundries
the main compartment is just one large one w/ a nice rolltop- if the material isn't waterproof, it appears very (very) water resistant