Multimedia and Heavy Computing
When comparing laptop form-factors, one of the key features to consider is the screen size. This may seem obvious, given that the form-factor is generally defined by and given in terms of the size of the display, and one of the biggest reasons to consider a larger display is for multimedia purposes.
It goes without saying that when watching a movie, the bigger the screen, the better. No one would bother spending $10 to watch a movie in a theater if that weren't true. While both the GX640 and GX740 have screens of the same resolution (1680x1050), the GX740's larger screen adds more to the movie-watching experience. The GX740 also offers significantly better sound quality, utilizing optimized speaker placement and the addition of an integrated subwoofer. Of course, this is all academic if you have an HDTV and intend on watching movies via HDMI, which both systems have. If you intend to watch any movies while on the go, you may even want to consider the smaller GX640, just to ensure that the battery lasts through the whole movie.
If a larger screen appeals to you for photo and video editing, the GX740 not only has a larger screen, but also gives you the hardware to make your editing experience even better. With optimized multithreaded programs, such as advanced video converters and 3D rendering software, the GX740 can perform up to 30% faster than its smaller sibling can. The same holds true with GPU accelerated processing, available in some video converters and prevalent in physics simulation software. ATI's own Avivo Video Converter runs up to 35% faster on the GX740 than the GX640.
If you're into multimedia, whether you like to watch it or edit it, the 17-inch form-factor of the GX740 makes a very good fit.