I have a Lenovo T61p laptop and when I do design work I usually run both monitors (independently).  Laptop is 15" (1680 x 1050) and second display is 22" (also 1680 x 1050).  Laptop runs a dedicated graphic card (NVIDIA Quadro FX 570M).  Operating system is XP SP3.  I never had any problems with that setup, even with demanding programs (for example I will often use AutoCAD 2008 on the laptop screen and SolidWorks 2008 on the external display at the same time, and never run into problems).