To the best of my knowledge neither ATI not nVidia are talking about new cards yet (except ATI's answer to the 7950.) The only real reason to wait is that none of the current cards are DX 10 compatible. But since DX 10 isn't coming out till next year, and games will (probably) support DX 9 for a couple more years, you might as well buy now. In 3-4 months the next generation of video cards will probably be announced but still unavailable, and prices on new video acrds don't go down very fast anyways. Best to buy now.
You should probably consider a 7900GT, it's more expensive but the performance increase is pretty good.