well, as far as waiting to purchase, if you need the card now, i would purchase whats already readily available; the 7900GT over the 7800GTX, because they perform about the same, but the 7900GT is certainly cheaper ~$300... as far as estimated release date and rumored specs of the GeForce G80 Series:
Confirmed Information
The GeForce 8 Series, or G80, will be the eighth generation of NVIDIA's GeForce graphics cards. Little is currently known about this series other than the expected release should be in late June to July of 2006. The memory interface is said to be GDDR4 memory, along with support for Microsoft DirectX 10, and Shader Model 4.0.
Rumors
48 Pixel pipelines might be implemented on the new series.
The new flagship card might feature XDR RAM
Rumors have been floating around the computer industry of a Unifed Pixel Pipeline set up for the G80.
The series is also rumored to have fast memory and core clocks, and may be built on 80 nanometer technology.
http://en.wikipedia.org/wiki/GeForce_8_Series