Which laptop is better for gaming? Msi cx61 0NF i7 or msi cx61 2oc i7 or lenovo z510?

The Lenovo Z510 with i7-4702MQ (that's the 4th gen i7).
It's got a 740m. However, there's 2 editions of the 740m: one is 64-bit, the other is 128-bit. If you can find out which version it is then this may help you to decide.
The 645m is always 128-bit, as is the 745m.
If you can find a 745m or 650m then this would be even better.
Don't get the 720m, you'll only be disappointed.
645m (128-bit) vs 740m (64-bit):
http://www.game-debate.com/...m-1gb-ddr3