As far as bags go, the Crumpler Customary Barge is an awesome bag and great for hiking and whatnot. Hold lots of gear, fluids and your lunch, so it'll take pretty much anything you want to throw in it. I think it also holds a laptop, could be wrong about that. A good friend of mine swears by it for her nature and travel shooting. If you need fast access to gear, stick with a shoulder bag.
Personally, I'm a big fan of the Kata backpack series, I'm leaning towards the R-103 for this coming summer's photo expeditions. It's well-padded, has good access, and is built like a tank.
On to flashes! The SB-400 is a waste, don't bother. If it can't swivel, it's not worth having on-camera. The biggest difference between the 800 and 600 is power and features - the 800 mas a higher maximum light output than the 600. It also has modes like RPT that has the 800 act as a triggered, timed strobe of various frequencies. The 800 also has the big advantage of being able to control other 800's and 600's in the wireless CLS system. Your D80 has the built-in ability to control off-camera CLS flashes, so the SB-800's feature may be a bit redundant for you. (caveat: IIRC, the D80 can only control one group of flashes, while the SB-800 can control 3.) The 600 also doesn't have an external PC jack for connecting to studio strobes.
So, really, it's up to you whether you think you need the 800. Most people don't need the features it offers. Me, I bought the 800 and I love it. It's powerful, flexible, and has all the features I need for what I do. For 90% of people, the 800 is overkill and not worth the extra $, as the 600 is a perfectly capable flash.