like alienware says on their website they have the fastest 15" laptop by customizing it to the max.but it costs quite much.$$$$$$$$$$$
Fastest...... doing what? Encoding video and audio? Or are you interested in gaming computer? Fast = expensive. What is your budget? For the best bang for the buck you'll want to spend more the GPU and less on the CPU.
Actually AW M15x isn't the fastest 15" laptop. Sager NP8690 is currently the fastest 15" laptop with 5870M/GTX 460M.