Welcome to the wonderful world of gravel grinding.

Dirt road riding is becoming more popular with the mountain bike crowd and as a result a new bike category has been created--the SUB. Salsa has been on the forefront of creating bikes for the gravel roads and they introduced the Fargo and it's sister bike, the Vaya.
If you are looking for a lightweight bike, perhaps a titanium
Fargo or
Vaya? The Fargo is technically a touring mtb that is designed primarily for the trails and gravel roads whereas the Vaya is more of a pavement to gravel type bike.
I ride on hilly gravel roads and the Fargo with a triple is my go to bike even though it weighs in at a hefty 31 lbs with the rack and fenders. With the heavier bike it smoothes out the ride and reduces fatigue.
The Surly Crosscheck would be another good choice for gravel grinding.
If you are unsure about the size that is appropriate for you, I would contact the company directly if your bike shop doesn't have the bikes available to test ride. With the sloping top tubes on the Fargo and Vaya, there is wiggle room in sizing. I usually ride a medium size road bike but in the Fargo I ride a small.
Good luck with your search and let us know what you decide.