Y500 has an ultrabay; you can swap out the second gpu and put in an optical drive if you purchase such an ultrabay opticaldrive. And obviously you can just jack an optical drive into the usb ports.
I think the sli 650m's are better than the 660m since the gpus in the 650m and the 660m should be the same and it is just a clock difference (maybe an extra 90 mhz)
My guess is that the frame rate difference for the sli 650m would be 50% more than the frame rates for the single 660m, since you don't get perfect doubling of frame rates in a sli configuration (and accounting for the clock difference).
But I have yet to see any benchmarks for the 650m sli.
I would also look into whether the y500 has optimus. I've heard some speculation that it does not implement optimus.