I7 Processor or 3 GB graphics card?

The Nvidia 555m isn't powerful enough to utilize 3GB worth of VRAM; the 1.5GB card is plenty. In short, Dell only put that there to try to trick people into spending money on a useless upgrade. You should get the i7-2630 as it will make multitasking easier. Even AutoCAD can benefit from multicore in some situations.