There is one glaring difference between the two you have listed... the Dell is running Windows 7 - 64 bit, and the Asus is running Windows 7 - 32 bit, so the Dell would be better. On the graphics side, the Asus is better, so there is a trade-off. I would go with the Asus for gaming. These websites are awesome, and will allow you to directly compare the two graphic cards.
ATI Radeon 550V:http://www.notebookcheck.net/ATI-Mobility-Radeon-HD-550v.31270.0.html