If you're planning on just doing some light, casual gaming, and don't expect to play upcoming 2012 games on high settings, then a GT 540M would probably suffice. If you're planning on using it to game for a couple of years... I would suggest raising your budget and going with something a bit better.
You should check this out:
Computer Games on Laptop Graphic Cards - Notebookcheck.net Tech
It'll give you a rough idea of which graphics card would be most suitable for your needs based on what type of games you wanna play.