I think the theory is pretty simple: they can overheat because with such fast cpu's you push the cards into 100% load ALL the time. And that would be a task even for a more modern graphics card that is passively cooled. And voodoo2 (most of them) do not even have a heatsink.... AND sli is 2 times the heat.
So yeah, always cool your voodoo2 cards, even in slow systems. I don't think there will be any problem with fast cpu's if they get good cooling.
asus tx97-e, 233mmx, voodoo1, s3 virge ,sb16
asus p5a, k6-3+ @ 550mhz, voodoo2 12mb sli, gf2 gts, awe32
asus p3b-f, p3-700, voodoo3 3500TV agp, awe64
asus tusl2-c, p3-S 1,4ghz, voodoo5 5500, live!
asus a7n8x DL, barton cpu, 6800ultra, Voodoo3 pci, audigy1