Looks like a pretty kick shiz machine... if you're not afraid of dropping that kinda cash.
As whitebox machines go (yes, Alienware is technically a whitebox computer. They're now just part of the Dell universe) it is probably one of the most capable machines you'll find. With dual-core processors, SLI graphics and DDR2 memory, these things will for sure be able to play anything you throw at it, but for me it comes down to whether I really need that much machine. Personally, the added framerates I might gain with this computer aren't worth the extra $$'s.
I game on a somewhat aging computer right now (important specs in my sig), and there's really only been a couple games I've ever had noticeable framerate issues with. And those were actually not typically in the genre of games I really enjoy so it hasn't been an issue. Elder Scrolls Oblivion was the main one I can think of and I didn't really find that game all that entertaining so it ended up being a non-issue.
My main requirements for upgrade at this point is a dual-core processor and a DX10 capable video setup (the dx10 gaming age is almost upon us). This machine definitely delivers on both of those points, but unless you REALLY want to own an Alienware, you could most likely build yourself a similar machine for significantly less than 5k. There are also a couple other companies that build "gamer" machines that you may want to look at as well.
VooDoo PC is one that comes to mind.
I can't imagine that anyone would be unhappy with this rig, but I can see someone being a bit regretful at having spent that kinda cash on a PC. I'd personally rather buy a new motorcycle and live with my slightly aging computer that still does a darn fine job for my gaming needs, but that's just me.
