Brandon, single image renders work best with RAW files since it has all of the data to work with.
HDR look the best, IMO, when they are shot with at least three exposures with an ev range from +2 to -2 to cover all the possible exposure ranges, and some type of dramatic lighting is a huge plus as well.
Now, the nose art pic I posted earlier is an HDR, there wasnt real dramatic lighting in the shot, but I attempted to get as much detail in the rivets as possible.
Keep playing with it you will get it!
Oh and check out Trey Ratcliff, he does some amazing work I think so anyway!