It depends on what you want to do. If you are primarily going to play games do other relatively simple tasks, then the dual core will be fine. A quad core may help boost gaming performance by a little bit, but most games are still GPU dependent. Speaking of GPU, the GT 540M is not that powerful, but it can probably last you a while if you eventually do not have a problem with playing future games using low graphics quality. You will be disappointed sooner with the performance of the GPU than you will be of the CPU.
If you plan on doing CPU intensive tasks like 3D rendering, or video encoding, then a quad core CPU will be the better option.
I just want to keep my laptop for a long while. I figured maybe a quadcore would be best for that reason. I know the 540 isnt that great but if you over clock it it can do great things. Besides my resolution is so small gaming is great. My friend has last gen i5 and games run pretty well. i never play really new games unless its on my desktop
If you are not going do get rid of the laptop as soon as you are disappointed by the GT 540m's performance, then getting a quad core can extend the overall life of your laptop outside of games. However, if you are only going to use your laptop to surf the next write some papers, and watch videos, then getting a quad core might be overkill.