What should I buy for a dual sport bike?

[Juggernaut]

This will be my first dual sport bike and I was wondering what I should get. I'll be riding mostly in trails/fire roads, and I'll be doing some riding on the highway. I'm not looking for anything like enduro, I'll be riding at most 100 miles at a time on the road. So what is a good dual sport bike that is more trail oriented? Should I buy a regular 300-600-cc and get a kit?

I forgot to mention I live in Massachusetts. Very tough to make a bike designed for just off road street legal here.

[Firestormer]

Yamaha WR450f. Can be road legal, will handle the highway ok and not too big and heavy for the trails.

[TheFreakShow]

My DRZ400 is the supermoto model so I can't really talk about its ability off road. It's actually not too bad on a good packed gravel road though, and it will hold highway speeds for hours without complaint. I think I'm getting right around 55mpg without trying to be efficient or easy on the throttle. There are probably several great choices out there, but a DRZ400 is worth looking at. They are really inexpensive used too.