Getting in to PC gaming... Alienware X51?

Hey guys, I'm a on-again off-again console gamer looking to into PC gaming. (I currently have a PS3) Skyrim and Diablo 3 are the 2 games I have in my sights for the near term. I've built many PC's over the years and enjoy it, but I've never really built a gaming rig before. Sounds like fun, but the Alienware x51 seems very intriguing to me. I love the form-factor and the fact you get pretty decent bang-for-the-buck.

I've spec'd out a few mid-size gaming builds on newegg and I could certainly build something a little faster for a bit less money than the x51, though not dramatically so. My only concern with the x51 is the upgradability of the GPU. I've heard anecdoctally that as AMD and Nvidia come out with new GPU's, they tend to get more efficient in terms of power usage - the theory being that even though the total power support on this thing is 150W (for the GPU) that might be enough to power the next generation of GPU's. I'm not sure if I should count on that happening, or if that's just wishful thinking?

Anyway, just looking for some opinions from the crowd. Is the Alienware a decent machine? Should I be concerned about the GPU upgradability? If I did build my own, my goal would be to spend around $1000 or less. (I'm not looking to build the biggest baddest machine in the world, just something that will let me play the latest games at relatively high settings)

Thnx!