The following is pure speculation:
Vista ships with drivers for the nforce3, but the drivers probably will not perform as good as the official nvidia ones do in XP 32bit. (due to a less mature AGP GART driver amongst other things)
There will be no more official drivers for the nforce3, although, the nforce4 drivers "might" work, they have been known to work well on nforce3 chipsets in XP64.