Here's where I got the idea for mine. There's a few minor changes I made, but not too many.
http://www.lincolnelectric.com/knowl...ontent/bbq.asp
On mine the right 3/4 is grilling surface and the left 1/4 is a griddle, for cooking up bacon or eggs to top burgers. When I'm smoking, I remove the griddle and build the fire there, and keep the meat on the grill. A pan of water under the meat keeps the moisture. The smoke travels across the meat on it's way out the chimney.
This is for rather large items. If you want something smaller you can use any grill as a smoker. Just set the coals and wood chips off to the sides of the grill with a pan of water in the middle.
oh, and for heat I always use charcoal and wood chunks. You can use gas if you get a box for wood chips.