The Y480 will be faster. However, as a word of warning from someone who owns a Y470, these laptops tend to run a little hot while gaming. Mine typically gets up to 90C - 92C while playing games with the exception of GTA 4 which hits 99C and the CPU starts to throttle.
Based on some pictures and posts by other Y470 owner in other forums (not THG), Lenovo uses an excessive amount of thermal paste. There is so much being used that it starts to act like an insulator. After wiping all the original thermal paste away and applying some Arctic Silver 5 or IC Diamond or other thermal paste, the temp drops by 20C.
I haven't replaced the thermal paste in my laptop yet 'cause I don't really use it to play games, but more importantly I bought a 2 year warranty extension and taking apart my Y470 to replace the thermal paste will void my warranty. I'll probably end up calling tech support to complain about the high temps and try to convince them to reapply a lot less thermal paste.
Maybe Lenovo figured out they are using too much thermal paste. Maybe not...
Other than that, the Y470 (GT 550M) is a pretty good laptop for games. The Y470p is better since it has the Radeon HD 7690M. The Y480 will be even better because the GT 640M is even faster. But if you have an option to choose 1GB or 2GB of VRAM, then just go with 1GB since the screen resolution is only 1366x766. You generally do not need more than 1GB of VRAM unless you play games above 1920x1200 resolution.
BTW, Ivy Bridge CPUs like the one in the Y480 tends to run a little hotter than Sandy Bridge CPUs. They use less power than the 2nd gen Sandy Bridges, but they also have a smaller surface area which means they cannot transfer heat to the heatsink as well.
Just be cautious about the CPU temps. I didn't really bother checking my temps until I decided to write up a review for the Y470 a month after receiving. Too late to return w/o for a full refund. But since I don't really play games with my Y470 so the high CPU temps are more of an annoyance than a real concern.
As soon as you get your Y480 install Core Temp to monitor the CPU temp when playing games. If you get 90C+, then I suggest you return it for a refund. Excessive heat can kill a laptop and 90C+ is considered excessive. Or you can replace the thermal paste yourself, but that will void your warranty.
I have the Lenovo Y470p and am 100% satisfied with it. The temps run fine, max of 70c when running skyrim on high.
Just a word of warning though, there are no updated drivers for the Radeon HD 7960 since it has the whole switchable graphics thing. AMD does not support it. I tried installing the ones from Lenovo's website from April and it caused all my games to stutter. I had to do a factory reset to get the original drivers back. I would go with the 480 since Nvidia actually supports their mobile cards.