You'll probably end up waiting anyway. The more powerful your computer, the more tempted you'll be to compose longer and more complex scenes. It's a vicious circle. :)

That said, Alienware makes some very nice high-performance laptops. The Area-51m Extreme can be custom-equipped with a Pentium4 3.2GHz processor (with HT technology and a 2.5MB cache), 1GB of PC-3200 memory, ATI Mobility Radeon 9600 Pro (128MB), Audigy soundcard, 160GB of hard drive storage, a DVD burner, and built-in Ethernet for about $4000. Alienware also sells dual-processor workstations bundled with 3DS Max 6. The MJ-12 series might interest you.