in XP, FROM WHAT I'VE READ, you can. XP supports multiple video drivers. So yes. FROM WHAT I'VE READ on vista they all need to be the SAME driver. IE all ATI, Nvidia, same model intel, etc. So no.
The new driver model for Vista should continue in W7, although I don't know this for a fact. The current state of affairs is that you can mix the types of ATI or NVIDIA cards but you can't mix ATI and NVIDIA or Intel. You must use one driver, like all Catalyst, etc.