3 Screens

[Delatory]

USB on a screen? Lag much?

Depends what you use the third screen for.
If it's gaming you'll have no chance. If it's for general perposes ie. having explorer open and msn and other stuff, it won't be a problem.

[Stephen!]

It will also depend on the resolution

[The Professor]

Aparently USB monitors are a horible drain on your computer because your computer is powering and rendering everything on it, you won't able to run it effectively on anything but a high end machine and if you have that kind of money to burn you may as well go for a proper screen and graphics card.