Video cards for laptops are typically sold only to OEMs like Dell / Alienware / Falcon Northwest, etc and they would install the card in the laptop's MXM slot. If it has a MXM slot. Most laptops have the video chip soldered onto the motherboard. These laptops cannot be upgraded.
If your Alienware m15x does use a MXM format video card (type I, II, III), then it is possible to upgrade. However, finding a place to buy a MXM card is the difficult part.