VideoCardz.com, a website that frequently reports on graphics card rumors, has posted that the Nvidia GTX 880 might arrive next month at Gamescom.
The website points out that multiple sources are pointing towards a mid-August launch, and includes a forum post from NotebookReview.com. This post is made by user800, who appears to have signed up just to make this post. Included in the post is a screenshot of a 3DMark FireStrike extreme score, which is a higher score than the GTX 780 Ti achieves. The model name of the graphics card is blurred out, but there is a consensus that this is set by the upcoming GTX 880 graphics card.
Nvidia also posted on July 17 that it was "very excited," and "T-minus 27 days." Curiously, that corresponds to August 13, the first day of Gamescom. A quick peek in the vendor list for Gamescom also reveals that Nvidia will have a booth.
We suspect that Nvidia will announce the GTX 880 and GTX 870 at the show. They are reported to be GM204 based cards.
AMD will also have a presence at the show, although we haven’t heard any rumors tying an AMD graphics card to Gamescom. The rumored Tonga GPU is expected soon, but nobody seems to know exactly what "soon" means. Mid-way through May it was expected that a Tonga GPU would arrive within three months, more or less coinciding with Gamescom as well.
All of this is just speculation for now. Vendors do make these announcement at big events, but not always.