Depends on what operating system you use, and what setup you choose.
You can do spanned single display mode, which implies the color depth must be the same, and the resolution should (in some OSes, it has to). Then there's true multi display mode, where you can do as you please.
Note that NT4 can only do spanned mode with identical resolutions, and Windows 2000 can only do spanned mode on single-chip multihead cards (with the exception of Matrox G450/G550 cards, whose drivers trick W2K into doing true multihead).
regards, Peter