Jerome -
While CAD and higher-end 3D applications will do the job, the learning curves on both are pretty steep. I suggest something cheap and easy:
http://www.individualsoftware.com/products/home_garden_design/total3d_home_...
Total 3D Home Landscape will let you scan in photos, then place libraries of plants, trees, and even buildings, to give you a quick visualization of your plans. I believe it's about 40 bucks. There are a couple of others that might do the trick - just do a Google search on "3D Landscape".
Joe Bourke
Art Director / WMUR-TV
