The vast majority of laptops do not allow you to upgrade the graphics card. The few that do tend to cost at least $1,500+. Of course don't simply assume that if you buy a laptop in that range you will be able to upgrade.
Desktop and mobile graphic cards are totally different in size, format and performance. Laptop graphic cards are smaller, follows the MXM I/II/III format and are less powerful than their desktop counterparts. For example, the Mobility Radeon HD 5850 will roughly equal to the desktop Radeon HD 5770.
You should be able to play GTA 4 @ 1600 x 900 resolution with the nVdia GT 560M. Watch out for heating though since GTA 4 does really stress the CPU. Whatever laptop you end up buying be sure to research any overheating issues.
There weren't really any professionally written reviews of the Lenovo Y470 when I bought it. Afterwards a few came out, they noted CPU temps as high as 92C (i5-2410m), but they didn't worry / care about it since games ran stable. Had I known about the issue I would not have bought the Y470. Sure enough, when I tested games like Fallout 3 and Crysis, the CPU temps ranged between 88C - 92C. The nVidia GT 550M ran relatively cool at around 61C - 64C.
Playing GTA 4 on the Y470 caused the CPU to eventually start throttling (causing stuttering). Upon looking at Core Temp (program I use to monitor the CPU temps), the CPU hit 99C.
When playing games on my Y470, I turned off Turbo Boost so that the CPU does not overclock itself. That brought down the max temps to around 85C, but it defeats the purpose of having Turbo Boost and it is still a bit too high for my liking. I prefer the max CPU temps to be around 75C.