While not aimed spesifically at home/garden design,
Sketchup is a great piece of software, which can be used for both purposes. There are tons of materials and components to be downloaded/imported and a good community, too.
It has a steepish price of $475, but the 30 day demo is fully functional.
IMHO, it's the most hands-on 3D app I've ever used, it just feels great.
J