The problem with the actual laptops is that if you can have a more powerfull processor, more memory and hard disk, but you have to sacrifice the display resolution, some connectivities : no more ExpressCard, no FireWire, no eSata, sometime a burner, etc...
My "old" Asus M51Vr has a 1440x900 display with all the above ports. But its weight is 2.8 Kg with a small battery.
In addition to what Oliver renault mentions, you have to decide what is your priority : weight and battery life for mobility or if you mainly will use it at home.
If you decide for the first option you may choose a laptop with ULV (SUxxxx) processor with an Intel graphic processor, or best, a dual GPU : Intel 4500HD + NVidia or ATI, so you can switch to one or the other depending of your need (Asus UL50G-XX035V or the like).
Some Laptops have the same mechanisism with a core i3 processor such as the Asus K52JR-X5 or the like. Battery life could be between 8 hours to 12 hours depending of the use, with the standard HDD.
This capabiility does not exist with Core i5 Laptops, as far as I know, or at the very high cost.
The weight may vary depending of the battery : between 2.2 and 2.4 Kg.
Even if I chose the MSI-X600 with SU9600 laptop, I can't recommend it as first choice as it has only an ATI Raeon HD 4330 GPU and this GPU draw much energy from the battery. So a 4 cell battery will give your at most 4 hours in Eco Mode, and about 8 hours with a 8/9 cell battery, again in Eco Mode.
What decided me for this model, is that it is quite thin and very light : 2.1 Kg (I guess the lightest one in his category and his size), and because I have another smaller Laptop or a 7" Tablet for mobility...
