http://www.cyberpowerpc.com/ They are reasonably priced for the same specs than Alienware. Also, whichever boutique you go with, do not order online. Always haggle with a sales rep. If one rep doesn't give you discount, hang up and call back. Somebody will give you a discount.
Definitely build your own if you can, I just built my first rig almost 2 months ago and I learned by watching a youtube video lol. Also Alienware sucks just from experience with friends who owned alienware in the past.
My gaming rig is from them. Other than a slow shipping date, prices were about $100 more than if I bought them myself without waiting on deals for the lowest price possible. As for PC gaming, I suspect it will be better than ever since next gen consoles will be running on PC based hardware.
If you're buying Alienware you're just buying it for the brand. If you want a reasonably priced gaming PC you might as well build it yourself. There are custom PC building services that are cheaper than Alienware.
While I mostly agree with building yourself a nice rig (I've built a few), from a straight form factor standpoint, there is almost no competition in the size/performance that the X51 brings especially with a GTX 670. I'll admit, I want an x51 under my TV soon.
Here are some alternatives: Velocity Micro: http://www.velocitymicro.com/PC.php Current new: http://hothardware.com/News/Velocity-Micro-Now-Offering-CustomBuilt-PCs-With-Haswell-Inside/ Lotus: http://www.lotuspc.net/us/astra.php Puget: http://www.pugetsystems.com/ Digital Storm: http://www.digitalstormonline.com/
I got one from velocity micro and have been happy with it. My brother bought one from ibuypower and it was clear that they had not even tested it to see that it worked. The components did not even fit properly in the case, and the fan would get caught on various wires. He sent it back and just built his own.
I'd expect more/better PC versions, if anything. PC ports used to be pretty difficult and time consuming (porting code from say Cell/EE), but given setups for the PS4 and Xbone, it should be fairly easy/inexpensive to get a PC port released. Of course, you'll see more console versions of PC games as well...which is either good or bad depending on how you look at it.