Another question about 2nd monitors - does it affect anything else? It seems like having two monitors would slow the graphics card down, and thus slow down things in the system? How does that work?
Essentially exactly like you said. If you really have a lot of windows open, I guess it's more likely that you'll see little glitches in screen rendering (like windows turning white while you move them occasionally, etc), because you run out of VRAM. Although that should be unlikely if you have 256MB of VRAM. Also you'll probably end up pushing the GPU harder purely because you'll be doing more stuff on those two displays.
Overall, in most uses, though, the impact is pretty negligible... running Aperture on both screens is probably a different story.