I'm guessing your video card. I recommend getting a Nvidia 460 GTX. It's cheap and powerful as hell.
LAPTOP. HE HAS A LAPTOP. STOP RECOMMENDING THE GTX 460 BLINDLY AND READ THE DAMN POST. HE HAS A LAPTOP.
DO YOU UNDERSTAND?Anyway, for a laptop you have very solid specs. Unfortunately, you have a laptop. DiRT 2 requires a higher end computer to fully utilize the DirectX 11 effects.
System requirements
Recommended Specifications - Enhanced for DirectX 11
OS: Microsoft Windows Vista Service Pack 2 or Windows 7
Processor: Intel quad core or AMD Phenom II X4 955
RAM: 3GB
HDD Space: 10GB
Video Card: ATI Radeon HD 5750 or higher, NVIDIA GeForce 400 series or higher
Mobile hardware is quite a bit weaker than desktop hardware for power consumption and heat issues. You're either going to have to disable DirectX 11 or lower the general settings. Sorry.
There's a lot more to a video card than the amount of RAM it has.
And I'm not familiar with 3dmark scores
Yes, video RAM is really only important when you have a larger monitor (1920x1200 or above) or are using several monitors like with AMD Eyefinity.