The Alienware is bigger, heavier, and doesn't have the same battery life of the Voodoo. The Alienware has a desktop processor, so this would explain it. The Voodoo is thin, light, Pentium M (fast), and has a 128 Megabyte ATI 9600 Pro Turbo. I think the Alienware is only a 9600 128 Megabyte Pro. The Sentia doesn't even compare, and I still can't figure out why a gaming company would release a notebook with Intel Extreme graphics.
If you were going to compare the Alienware with a Voodoo "desktop replacement" the 855 would be a good comparison. I personally think the 855 will beat an Alienware in performance based on the 3400+ Athlon 64. If you want thin and light, go 460. If you want desktop replacement go with the 855. IMHO.