Well, the first thing that comes to mind, is you want a desktop system, not a laptop.

That all being said, the answer is always the same with laptops: Get the best possible laptop you can afford. Get the fastest CPU with the largest number of cores, the most RAM, largest hard drive, biggest screen, etc.

Video editing will live and die by three main factors though.

1: CPU speed
2: RAM
3: Hard drive speed

Which is why you don't want a laptop. Mobile CPUs are not as powerful as their desktop counterparts, laptop RAM has fewer pins for reading/writing data, and hard drives are slower. So you're getting screwed in all three areas where it counts the most. But if you insist, see the above about getting the best possible laptop you can afford.

Also, make sure that you're using a codec that is multi-threaded, like x264, so that you can distribute the encoding load across multiple cores. Some encoders can even utilize the GPU for encoding video data, which is REALLY nice since GPUs are designed for stream processing, and that's EXACTLY what video encoding is.