I noticed some issues with a second monitor connected when playing Hitman Blood Money. Even if I disabled it with UltraMon when playing, it was fixed as soon as I disconnected the damn thing and started to use 1 monitor again.
If you have a video card with dual outputs, then you can have a dual monitor setup. You can mix and match monitors, such as a 19" LCD and a 17" CRT, however I know from experience that unless the monitors are the same or fairly identical, it's not a good experience, since one monitor could be at a different level, one could be smaller, ect. But technically you could use any monitor you so choose.
As for the lagging, that will only be an issue if your running an extended desktop while playing a new game with a not-so good video card. Since you would be running a much larger resolution, it may severly cripple game performance. However for normal tasks, it should run just fine.