Bently, I just started using Kerkythea, and it does a pretty good job. I’m no expert…I’ve only been playing with it for about a month, but even with basic settings, its a better visual than sketchup alone. The software is freeware, and there is a pretty large community where you can find different texture packs, and learn how to modify textures directly out of sketchup to give them a more realistic look. Take a look at this example:

With the right person behind the computer, it could look much better than what I did in just a couple minutes of playing.