Nvidia is planning to launch the GeForce 9500, and with the launch of that card it will complete the cycle. We don't know much about the GPU, but it looks like we are talking about a refubrished 8500 all over again.
This will be a low-cost alternative and as people think that higher numbers means that it's better, they will go for it.
So the new lineup will look like this. GeForce 9500 is the low end, 9600GT is mainstream, GeForce 9800 GTS is Performance, GeForce 9800 GTX is high end and finally the GeForce 9800 GX2 is the ultra high-end.