I bought the M17xR3 and had the same exact question about DDR3 1600 Memory with the i72630M. My research determined that you can infact use DDR3 1600 or even DDR3 1866 Kingston HyperX Memory with the 2630M, BUT... it will default down to the DDR3 1333 Speed settings. This has its advantages though with a couple of ways.
1. Dell is way overpriced for 16GB of Memory. They want $360, but New Egg is as low as $200 so you save money.
2. The Kingston HyperX Series comes with Heat Shields
3. Having 16GB of DDR3 1600 or 1866 Memory in your Alienware now, means if you ever come across a good deal for the i72820 or better later down the road, then you already have compatable memory in place.
I talked with several Dell Reps and Alienware Techs, the majority of them are stupid with in depth tech knowledge. But sometimes you get someone that actually games or builds PC's and they will tell you exactly the type of Ram they prefer.
Dell by no means makes their own memory!!! But the Lame operators will tell you that you can only use "DELL BRANDED" memory or you will void your warranty, NOT TRUE!!!!
The Memory slot located near the Hard Drive Bays are user upgradable, meaning that you can upgrade them anytime you want too as long as you use the recommended specs by Dell. The Memory slots that require you to remove screws to get under the keyboard however will VOID your warranty if you remove them. I was also told the so called "DELL BRANDED" memory is actually "SAMSUNG" by one of Alienwares custom builders in the Tier3 Tech support.
So bottom line. If you order an M17xR3 or an M18x with 4GB min of their memory, it will be 2 x 2GB of Samsung 1333. Then go to NewEgg.com and buy 8GB KingstonX sets of DDR3 1600 memory from $94 to $110 each set. You save over $150 by not getting is pre configured by Dell.