Which Would You Choose, 17 or 15?
Choosing between a 17-inch laptop and a 15-inch can be very difficult, especially when you seek high performance paired with mobility and battery life. In the case of the GX640 and GX740, both suffer from a relatively short battery life, as is the case with most performance laptops. If battery life is your main priority, you may want to consider a non-performance alternative; however, if battery life is something you're willing to sacrifice but still want as much as possible, the 15-inch form-factor may be your best choice. Their screens use less power, and they are generally paired with more energy efficient hardware, as in the case with the GX640. There are also similar 15-inch laptops to MSI's GX640, like Asus' G51Jx and upcoming G53.
If a larger screen is your priority, the 17-inch form-factor is an obvious choice, and the level of performance that the GX740 provides is nearly unparalleled; however, if the price tag is too high, MSI also offers the E7405, which also has a 17-inch display and Radeon 5870, just like the GX740. Instead of the Core i7 CPU though, it incorporates the cheaper Core i5 found in the GX640 as well as cuts the hard drive space down to 320GB in order to save costs. You can expect gaming performance very similar to the GX740, and processing performance comparable to the GX640. Battery life will be most similar to the GX740 though, due to the larger screen.
Asus also offers a very competitive 17-inch model known as the G73Jh, which retails for $1599, but can be found for as low as $1199 on clearance to make way for an updated G73 expected later this month.