Well of course, Vista itself needs a better hardware. But it's hard to answer this. We've used 4GHz Athlon 64bit with 2GB RAM for testing and there was no problem - but of course, this qutie a good machine(although I think it is not a good idea run Vista with less than 2GB RAM).
Regarding Riya - there should not be any difference. The Riya support has not changed(only if there would be some Riya-driver problem).
Anyway, could you be more specific, what performance issues?