I am using Sketchup free version and I think Sketchup is AMAZING ! and very fast. I recomend getting a "sketchup for dummies" book and immerse yourself in it. I've used a lot of 3D programs and it's hard to beat Sketchup for quick architectural models. The free version (I use v.5) is limited on export file formats but you can export to Flux 3D which is another cool 3D program. Sketchup Version 6 added a feature called "photo match" which let's you model from a photo with even more features. The pro version of Sketchup lets you export with more file formats but it's expensive at around $495 american dollars. But I say if you are a limited newbie like me, use the free version to do all your modeling, then when done, open them with a pro version demo that lets you use it for eight hours of time with full features and save and export unlocked but after the eigth hours, it goes into disable mode.
Also cool about Sketchup is you can import terrains from anywhere in the earth via google earth , but admittedly they are not highly detailed terrains, only basic.