I have a set of XP and 2003 x64 VMs on a 2012 Hyper-V server that both work just fine, drivers and all, but yes - they must be at the SP2 level, and if you want to use the legacy NIC you must use Vista's x64 DEC drivers from an installed source. I would recommend XP x64 SP2 + Integration Components + regular (non-legacy) NIC and you will have no issues.
