I'm running winxp, with an x800pro.
I think maybe the 6.5s were slightly better than the 6.4s. But the NGO modified 6.4 driver was better than any of the 6.5s, including the other modified drivers and the 6.5 beta (there is no NGO 6.5 driver yet
). But now, in my own testing atleast, the 6.6 betas outperform them all. A modified NGO, or maybe zer0point (I think the NGO's are better) 6.6 driver could be very nice indeed, if the official 6.6 release keeps the performance gains of the beta.
And I wouldn't be rushing to run games on Vista atm, as virtually all games suffer a performance hit when running under it- give the new OS time...