In theory all current OS' are IPv6 compatible (XP SP3 included). However, to my knowledge there is no protocol level technology to connect IPv4 addresses from IPv6 and vice versa (except for tunneling, which basically is an IPv6 connection). So you don't want to disconnect your hosts from native IPv4 just yet.
Your DNS server should work fine dual stacked, since it does not depend on the underlying transport.
Regarding the proxy, if your firewall allows it, it works, if not, no.
A word of warning: I don't know where you are going with this, but BE SURE TO CHECK THE SECURITY IMPLICATIONS FIRST! IPv6 isn't just a larger address space, you have to relearn from scratch. I hope, that was explicit enough.