After years of building or having my edit stations custom built, I went down for a Dell Precision m6400. This is a powerful portable PC marketed as a desktop replacement. It is too heavy and big to shove in a backpack but it is an awesome editing unit.
Mine has Core 2 Dual 2.66GHz processors but the newer models (always something newer) have the i7 cpu’s. It also has an excellent GPU – the nvidia Quadro FX 3700M. Together they allow Premiere CS3 to slice through HDV like budda.
The added advantage is that you can hook up an external 1.5 TB data drive or raid config and a 1080p monitor (Dell has a good one for under $200!) and you’re in business. The only thing I would strongly recommend is springing for the Premium tech support. Then when you have an issue that usually takes days of research and grief to solve, you get one of their guys on the phone and he researches it and solves it for you – I’m not kidding. And if you encounter a serious problem, they come to your place and fix it.
After all the years of playing around with big, noisy, temperamental machines, this “desktop replacement” makes them obsolete.