Depends what you're using the computer for.
Running two displays will use more of the video card's memory. This shouldn't be an issue for gaming. There should be plenty of memory to run the internal display and a 24" (1920x1200) external display. A 30" might be pushing it, and if you're gaming at native res. on a display that large, I expect you'd most definitely see a performance hit.
My opinion is that with something relatively idle on the secondary display, the video performance hit while gaming should be minor with an external display 1920x1200 or smaller.
If you're doing something like rendering, on the other hand, where your video memory might be pushed to the max, you'd definitely see a performance hit.