yes a very good question! i am also looking for some software - but i need software that will be able to visualise the actual planting schemes (plants in detail) in borders rather than just the lawn, patio and wall layouts.
I use Dynascape. Its not cheap but I draw in the Dynascape platform then that it either to Dynascape colour and colour it all in, or take it to Google Sketch up, clean it up and produce 3d design from sketchup. The sketchup program now has a app from dynascape within the program.
This is how my output looks: http://www.johncavilldesign.co.uk/design.html
Sketchup is the best free design program
It rocks !!!!