Dell XPS Gen 2 vs. DRC (in a cleo chassis)

Ok....so I have been reading review, upon review, upon review.....

In all my reading, it seems that the XPS fares better than the Desktop Replacement Computers for some reason (gaming wise), even though it has the Pentium-M chip and the DRC's have the Pentium 4's with HT.........I am in the process of deciding between the XPS Gen 2 and some kinda cleo DRC (Alienware, Hypersonic, Cyberpower, Rock, etc)--but I just don't get the difference in the processors and how the, for example, Alienware or Cyberpower with a 3.6Ghz P4w/HT, 1gig ram, nVidia 6800 go ultra, 60gig7200rpm laptop can't perform as well as the XPS 2.13Ghz P-m, nVidia 6800 go ultra, so on and so forth.....

Can someone solve this delimma? Are the Pentium-m chips just as good with lower Ghz as the P4's pushing close to 4.0ghz? Is there some kinda scale comparison between the P-m's performance compared to the P4's?

Also, if I were to buy the XPS instead of the DRC, how quick are the games going to surpass it, respectively? I am on a Mac right now and dearly miss playing PC games!!! I just don't want to purchase a high end laptop and then in a year it be not able to play the most rescent games. If anyone can climb in this and speculate with me, that would be great!!

Thanks for any help you can give me!