Gaming is generally GPU dependent, so regardless of a CPU of your choice even with the weaker GPU option, so GT 555m 1.5GB (which really isn't such a bad card), you will still be able to play games, e.g. BF3. The difference between 1.5Gb and 3GB version isn't anyway drastic, so if you are not going to play games the difference of those 100-150 bucks isn't worth it.
For the CPU, i5s and i7s available for m14x significantly differ: i5 2430M features 2 cores and 4 threads while even the weakest i7 2670QM has 4 cores and 8 threads and twice as much cache. This pretty much makes such an i7 CPUs quite potent, while the i5 remains 'regularly' efficient (meaning no thrills). If you won't game hard and play around with overclocking your GPU and so on, the laptop should not overheat. You can always on the fly switch off the dedicated Nvidia GPU and run everything just using the CPU and the Intel integrated graphics built into the CPU, and hence gaining battery life and lower temperatures. You can also apply high quality thermal paste to lower temperatures even more, but in case of m14x that's quite complicated for a beginner. All of this means I would strongly suggest to get an i7. If you don't want to spend too much, already 2670QM will do just fine. A stronger i7 processor will deliver much higher efficiency and will be somewhat more time-proof. According to alienware.com the difference i5 2430M vs i7 2670QM, including the cost of faster memory, is 140 bucks. That's not that much, is it? Worth spending.
Edit: BTW, 2670QM does not support 1600Mhz RAM. There is a mistake there forcing you to upgrade to faster/more expensive RAM. Better call them and order the laptop by the phone. I mean, faster RAM will be somewhat faster anyway even if the CPU is stuck at 'talking' to memory at 1333, yet it seems you don't want to pay too much, so stay with generic 1333.
You may also want to invest in two other things: a 1600x900 display and an SSD drive. SSDs drives truly make all the difference. Choose the cheapest disk option when ordering the laptop and buy an SSD disk separately. It will be much cheaper this way.
BTW, if you want your laptop to run reasonably cooled, keep its back a bit elevated, as the air intake is placed on the bottom and sucks the air from below your laptop. You don't need a laptop cooler for that (laptop coolers don't really work, let's face it). Something like Belkin Coolstrip will do just fine.