there is a couple of posts here with similar questions
from what i understand your biggest concern is expand the wireless network, that can be done in two ways, by using a router as a repeater of the first one, this can be done depending on the router, some models doesn't have the option
the other way is install a bigger antenna in the router, 7db antennas will expand the network coverage alot, 200 meters or more
about the mixed network, what is your exact concern? put them on a single workgroup? share resources?
that can be done, the problem here is win7 cooperating, sometimes it refuses to work with xp machines