I think the Alienware M15x has the GTX 460m in their option. I know it's not given in their website but you can call the customer care reps and check it out.
I have some doubts regarding your VANTAGE scores: the fact that the M15x and the g51 have the same gfx but the difference is huge. How? (I thought 3d mark Vantage was GPU dependent, correct me if I'm wrong though)