There are people doing this... I tried it bu the HD2400Pro I had wasn't up to the tasd or the problem is OS related... on XP extended desktop (where each monitor is considered independent) gives very poor performance but spanned (where all attached monitors are seen as one by the OS) doesn't.

On Vista the extended desktop is supposed to have been fixed.

multiple gfx cards must use the extended desktop model but, as each card can be considered a primary GPU you may not suffer the framerate gimp in the same way as multiple desktops off a single card.