Dell has recently decided added the Nvidia Optimus support of the XPS 14, XPS 15 and XPS 17. They are based on the Calpella platform.
The Dell Studio XPS 14 comes with a 1GB GeForce GT 420M or 2GB GeForce GT 425M graphics card. The Dell Studio XPS 15 has a GeForce GT 435M GPU with 2GB of memory. The Dell Studio XPS 17 equipped with GeForce GT 435M graphics with 1GB of video memory or a GeForce GTS 445M graphics with 3GB of video memory .