Edwin Alejandro eb Mezeta aka AlexEbMz is a master in Photoshop, and he had a very good works. He specializes in virtual Car Tuning.
His works can be seen here, in his gallery, as well as videos in which he explainshow he does it.
One of his latest works is virtually tuned Saab 9-3 SportCombi. In the next picture you can see a image of the SAAB before and after:
Also, you can see how he came to final results in next video: