a gtx 275 and a gtx 260 can run on the same machine... but they can't be SLI-ed. They can be used to support multiple monitors, or a one can be a dedicated Physx card.
In Win 7 its supposed to work 'better' than XP or Vista.
As for Hydra, its a hardware based support, while Win 7 is a software based support. So their just different. I'm not aware of any tests done to compare the two.