If your talking about XP Mode in Win7 (which is basically XP SP3 in a VM in Win7), then you'll need 7 Pro or Ultimate, but it's more cost-effective to go with 7 Pro (making sure to get a full install disc of it for the off-chance a rebuild is needed) as the stuff Ultimate has that Pro doesn't (beyond having the Premium Games disabled by default in Pro, which can be fixed later) is all to do with locking out data and spying on users.