Today at CEBIT in Hannover, nVidia officially announced the addition of a 512mb version of the GeForce 6800 Ultra to it's product line, available in both AGP and PCIe, the later still supporting SLI modes.

Also announced was the AGP version of the cheap and cheerful 6200 GPU, though most of the readers of this site will not be particularly interested in this budget card.

Should you be in Hannover in the next few days, you could visit nVidia's stand at CeBIT, Hall 23, Stand B43. I'm guessing they'll let you take a look at their new baby if you ask nicely.

Hopefully some hardware sites have a few for benchmarking and I'll add some links later.

** Update **
bit-tech has a play with 1gb SLI over at their site.