I finally found out the legacy mode of my HP NC510F nic just concerns irq handling. so this is relevant for performance, but not for Xen VM support – which i still havent found out enough about. The NIC should be able to expose 8 vNICs for domUs but so far it had refused to do that.
About the legacy mode, I remember reading something about new irq handling for PC servers but I don’t have the facts yet. There’s just too many of these buzzwords like SR-IOV, VTd-IO and so on floating at the moment.