I normally lurk on these forums, but I had to wade in on this one. It always irks me a bit when I see people just throw out there the same old urban legends about PC Brands, especially Alienware (and yes, I do own one).
First up "Aren't they just expensive Dells". Nope. They are designed, engineered and created by Alienware, a company based in Miami that has Dell as a majority shareholder. Dell handle construction of the machines and support (which is a huge negative in their favor admittedly) and the acquisition of Alienware though enabled them to hit economies of scale they were unable to reach as a small boutique manufacturer. Incidentally, the original founders of the company formed OriginPC and that's a brand that fully lives up to all the myths surrounding Alienware today.
Second, Alienware notebooks are overpriced and underpowered. Not really. In fact, not at all. Sure you can build desktops cheaper than you can buy Alienware desktops, but laptops are a whole different kettle of fish, and a segment of the PC market all to themselves. Yes, when they were still a boutique outfit their laptops were ludicrously expensive (again, go look at OriginPC's prices). Today though that's absolutely not the case.
Rather than just state an (admittedly somewhat biased) opinion, it's worth looking at hard numbers. How do you calculate the "value" of a machine? It's actually pretty easy. Take the machine and all competitors . Benchmark them all with the same benchmark. Let's use Vantage here.
When you've got your benchmark scores (or looked them up in reviews online), divide the price of the machine by its benchmark. That gives you a price per point of benchmarked performance. Then you just need to weigh up the actual performance off the machine in your own mind against the actual monetary value of the machine.
AnandTech recently published a review of the Razer Blade, and compared it to other high end gaming notebooks. They included the prices and scores of some of the machines on test.
http://www.anandtech.com/show/5606/razer-blade-review
Here's the 'value' using 3dMark vantage as the yard stick
Razer Blade - 38 cents per point
Asus G74SX - 14 cents per point
Alienware M18x with SLI - 12.3 cents per point
Clevo X7200 - 16 cents per point
Alienware M17x - 17 cents per point
etc etc
The highest end, and most expensive, Alienware laptop you can get (the M18x) is actually the one of the best value gaming notebooks out there. It also beat out all the others in performance numbers.
The mid range Alienware, the M17x, is actually extremely close to its considerably more expensive competitor (the Clevo) in terms of value as well.
Once you measure the value of what you're actually getting and weigh that up against the performance in game that you're seeking, Alienware are actually *not* overpriced at all. They are very well in line with the rest of the market, and in fact beat it on value in many cases and performance in most. They are also cheaper in total financial outlay. The M18x in the test cost 2800 ish. The Clevo they used they tested separately and it came in at 3700 ish. The M17x on test cost 2000, and is a notebook that can run pretty much any game out there at High or Ultra settings. The same cannot be said for the 'cheaper' machines.
Getting back to the original posters question though, yes your Alienware *should* be able to run Rebirth without issue, unless EgoSoft really screw up. Mine can run Skyrim at ultra detail in excess of 80 frames per second, and I can't imagine Rebirth puts the same strain on a machine that Skyrim does.