Okay. The screen is 1366 x 768 so the fewer pixels on screen means I can use medium/high settings?
You should be able to play most games with medium quality settings and anti aliasing set to x2.
My Lenovo IdeaPad Y470 has a Core i5-2410M and a GT 550M. While testing Crysis I was able to get between 35FPS and 44FPS in the second chapter "Relic" most of the time. It probably dipped as low 25FPS and peaked at around 54FPS.
The GT 540M will probably get around 29FPS - 38FPS most of the time in Crysis. I tested less demanding games like Fallout 3 and Mass Effects on my GT 550M so they should run pretty well on the GT 540M too.
I might borrow GTA IV from my cousin and give that a spin on my laptop.