The only accessory that I would recomend would be the upgraded waist belt. It's great for holding a camera, snacks, compass, etc.
Just remember that if you do purchase the Crew Cab, you'll already have the NICE frame so you can just buy another bag to swap to. Not only does it give you added versitility, you already have the frame sized to fit you.
I originally got the 6500 with the upgraded waist belt. Great pack, comfortable to wear for long hikes deep into the wilderness. A bit too big for weekend trips so I've since gotten the Crew Cab. As I already had the NICE frame all I had to do was purchase the bag only. I've used the Crew Cab for weekend trips and I also use it as a day pack. Again, an excellent pack. Having both bags, I can set it up to fit the hunt: 6500 with or without Top, Crew Cab with or without Top. And the Top also dooubles as a day pack. Although I haven't used it for that yet...