YB, the price tag for a good single-P4 computer is between $2500-$4500, which is in average more expensive than dual-G5 Apple, which I expect to be in the $2700 area.
Nice try. The Alienware computer comes with 512 megs more memory, better graphics (256M nVidia card vs. 64M ATI card), and Dolby Digital 5.1 speakers, vs. the dual-G5 system. Check out apple.com yourself (click on Store, then the Dual 2 GHz G5 system).
By the way, the dual-G5 goes for $3000. Plus, I don't know how you arrived at an upper limit of $4500 for a "good single-P4 computer." It seems like you're just making up numbers as you go along.
Tenchu