We won't know that until they come out but I suspect October or early November.
I really don't think Nvidia is going to release a new card if it won't outperform the old card.
The ATI 5870 is a fast card, but the GTX 295 outperforms it in quite a few games and Benchmarks. The Guru3D.com just reviewed the 5870 in Crossfire and the GTX 285 in SLI even managed to win a couple of tests. Radeon HD 5870 CrossfireX test review
If your planning to upgrade soon you should wait.
The i7 975 is a waste of money, the 920 and a Good Heatsink will beat it for a lot less money.