Issues with Alienware Area-51m

[White-Bread]

Good Afternoon All,
My cousin and I recently bought Alienware Area-51 Laptops. P4 2.8 GHz, 512 MB RAM, ATI Radeon Mobility 900 128MB. We are both owners and subscribers to Planetside but the game play is way below par. There seems to be some type of lag throughout the process. For example, while running in any enviroment there is like a shutter or glitch every 2 or 3 seconds. It's quite annoying and makes playing a nuisance. Does anyone know if this could be an issue with PS or if it could indeed be our laptops? We are stumped and quite frustrated because we thought we bought top of the line machines (for laptops). Please help!!!