I use a 50-200 mm lens which seems to work pretty well in those situations that you described. You can try a 300 mm lens (Nikon makes a nice 75-300 VR lens if you have a Nikon body) that's relatively inexpensive and works pretty well. The only issue I know people have run into is that your pictures aren't quite as clear as they could be at 300 mm when you're hand-holding the camera. When you start getting that far out it's really nice to be on a tripod for the best shots.

This buck was about100 yards out when I snapped this photo.



This buck was about 50 yards when I snapped this one. (It's cropped and color corrected)



And these does were probably 75 yards when I took this one.