The GT 555M in the Lenovo Y570, while not as fast as the true versions of the GT 555M used in the previous generation Alienware M14x and the Dell XPS 17, is faster than the GT 630M (GT 540M).
However, the Y570 is not a particularly good laptop to buy in the general case, because it has a 15.6" 1366x768 display. 15.6" 1366x768 and 17.3" 1600x900 displays make things onscreen large, and tend to have very poor image quality due to low contrast. I recommend not to buy laptops that come with this type of display to those who have a budget that allows them to get a laptop that has a better display and still suits their needs, and who don't require larger text for eyesight-related reasons.
Consider this
HP DV6-6C53NR. It has a 15.6" 1920x1080 matte display, and an AMD Radeon 7690M XT which is as fast as the true version of the GT 555M and is faster than the GT 555M the Y570 comes with. This laptop comes with an i5-2450M processor, and in case the laptops you were looking at have Core i7 processors, keep in mind that the processor is not something you should prioritize over the GPU and the screen unless you have a particular need for a fast processor. Game performance depends more on the GPU than it does on the CPU.
And ensure that, if you get the HP DV6, that you go into the BIOS and change the switchable graphics setting from DYNAMIC to FIXED. Some games will not properly engage the dedicated GPU under dynamic mode, such as Minecraft and a few others. FIXED gives you manual control over which GPU is enabled, and it should ask you to change enable the dedicated GPU when power is plugged in and disable the dedicated GPU when power is unplugged.