Dell updated the M17x from r3 to r4 when they started using Ivy Bridge chips, the 3rd generation Intel Core processors. So you can tell it's an m17x r4 by looking for the "-3xxx" in the CPU model number. For example, the m17x base config CPU is Core-i7-3630QM, which Anandtech called a "healthy step up from last generation's i7-2720QM" (that was the Sandy Bridge chip in the Alienware m17x r3).
(You can also get Core i7-3740QM or Core i7-3840QM in the Alienware M17x r4.)
More importantly, "Revision 4" brings stronger GPU options. Here's a quick list of what video cards you can get Alienware in the m17x r4 laptop, with the important cliff notes from Anandtech's review to save you time. These are listed from cheapest to most expensive.
GeForce GTX 660M - "really is a fine starting point for gamers"
Radeon HD 7970M - "the best price/performance option...a notable improvement in performance over the $250 GTX 675M upgrade, which is just a rebranded GeForce GTX 580M"
GeForce GTX 675M - "there's really no reason to shell out an extra $50 for worse performance with the GTX 675M unless you absolutely have to stay in NVIDIA's ecosystem and don't want to spend up on the GTX 680M"
GeForce GTX 680M - "the big draw with the M17x R4...a substantial performance improvement over last generation's top end GTX 580M/675M, the kind of generational jump we haven't seen on the mobile side in some time."
Now, if you're on a budget or very value-conscious, it's a good idea to go with the 660M or 7970M for graphics. But before you buy, there are a couple things you need to know.
If you want the GeForce GTX 660M and the best M17x r4 price overall, just grab the base config, which will be the cheapest deal we have posted above. And if we have a coupon code for it, you should grab it fast. Alienware coupons actually don't come around that often. When they do, it's usually a fixed dollar amount, so the nice thing is if it's for, say $50 or $100 off, you'll be getting the biggest percentage discount by using it on the lowest-cost base config (as opposed to only getting $50 off a heavily-upgraded system at $2400+).
If you want the Radeon HD 7970M for its value, it sure seems like a smart move to us. However, Dell has been making it a pain to get this video card without a bunch of other upgrades. For example, if you go to the base config customization page and click "video card," they usually don't even offer the 7970M. Here's a screencap.
Most of the time, the only Alienware M17x config with the 7970M already has hundreds of dollars in extra upgrades built in that maybe aren't so important to you, but you can't de-select. This often pushes the list price well above 2 grand. So if we have a deal with Radeon HD 7970M posted above, and it's under $2,000, it's actually a very decent value compared to what Dell usually offers.
Another option to consider...a few of our readers have basically said "All I care about is the video card, so I'm going big on that, and screw the other upgrades." Obviously you can find a variety of opinions on how important 8GB v 6GB RAM is, or the boost you get from 1TB RAID 0 storage over a single 500GB HDD. But if you really don't care about stuff like that, and there's not a deal for M17x with 7970M under $2,000 at the time, we can't blame people for going through the cheapest config we have posted (definitely grabbing the coupon if there's one available), and just upgrading the video card from 660M to the coveted GeForce GTX 680M for $550. Which makes the cost $2,049 minus any current discounts.
m17x with 1080p and 3D display
Most m17x configs we see have 1920 x 1080 resolution. The base config starts with 1600 x 900, and the upgrade to 1080p usually runs $150. Definitely nice to get 1080p to make the most of the HDMI in feature.
The 3D upgrade costs $400 (or $250 when upgrading from 1080p). It's a WideFHD 1920 x 1080p 120Hz display with 3d glasses, and it usually costs $400 more than the base 1600 x 900 LED screen.
The m17x is actually the only current Alienware with a 3D display available (3D is not an option on the current m18x r2).
You've probably seen or heard of laptops that offered 3D display but required you to select a certain video card. At least that is simplified with this system. You can actually get the 3D upgrade on M17x r4 even with the base config that has the cheapest GeForce GTX 660M discrete graphics.
more M17x r4 vs r3 changes, if you're curious
If revision 4 of this 17-inch Alienware looks similar to Revision 3, don't be alarmed. It is actually built with the same chassis. As mentioned above, the bump to Ivy Bridge processors and improved video cards offer legit performance improvements. And the other big change is the addition of an mSATA port. So with that Dell is offering caching mSSD and mSATA boot drive enhancements a variety of standard, RAID 0, and RAID 1 customizations.
Along with mSATA, here are a few more changes in r4 from r3.
M17x vs M18x - what's the difference besides the size?
Well for one thing, the M17x is generally cheaper -- you can check M18x deals here -- and that's due to a couple key differences.
M18x has anodized aluminum chassis; M17x is plastic, though it has a nice soft touch finish
M18x allows for a dual GPU setup, but the M17x does not
With the larger size and metal case, the M18x weight starts at 11.9 pounds, about 2.5 pounds heavier than the M17x (starts at 9.4lbs).
Finally, both models top out at 1080 vertical resolution (M18x no longer has display option with 1200 vertical) so the display is no longer a reason to pay more for m18x. If you want the metal and dual GPU, go for it. Otherwise, you can get mostly the same guts and performance with M17x as you'd get with many configurations of the M18x, but for less money.