I went back and forth between these two laptops more times than I can count. I'm not a hardcore gamer by any means, but I do like to play the latest games from time to time. In my case, battery life wasn't really a concern since I rarely use my laptop away from a plug, so it was really no holds barred between the two.
On the one hand, the GX740's quad core CPU with hyperthreading was a huge selling point for me. I do a lot of video converting as well as 3D modeling, so 8 cores is pretty sweet. The size of the thing is pretty extreme though and takes up almost all of my available desk space, leaving little room for my mouse, lol.
On the other hand, the GX640 is very similar to my current laptop, the GX620. It's the same size, same design, same layout, so switching to it would be very simple. It still has plenty of power to be more than useful, but compared to the GX740 it feels like a big hit to take.
When it all comes down to it though, the GX640 just offers the best performance per price of anything I've seen. I'm by no means a big spender, so at the end of the day, I have to go with the best deal.