More and more GT200 information has started circulating and that could only mean that we're closing in on the final tuning. The chip was taped out just recently, which means that it's lagging the RV770 by a couple of months, but most importantly, NVIDIA doesn't have to launch a new chip today. It already has competitive chips on the market. While G92 and G92b (55nm) will not offer any architectural advancements, GT200 will. DirectX 10.1 is suppose to be one of them. We know for a fact that the chip will be in the 1 billion transistor range, some say even up to 1.3 billion, at 65nm.
Unlike AMD/ATI, NVIDIA is not expected to use GDDR5. Instead it will go for a more complex PCB with GDDR3, which means a wider bus instead of higher memory frequency. GDDR3 chips tops out at 1100MHz today. We expect GT200 to use these chips, and with a 512-bit bus it puts it on par with RV770 and its 256-bit bus and 4GHz GDDR5 memory.
The clusters have changed from 16 shaders per cluster to 24 per cluster, most likely MADD + MUL, and 10 clusters with the high-end part, which means 240 shaders all in all. These will be accompanied by 32 ROPs, 120 TMUs and 120 TFUs. The core clock will hopefully be in the area of 600-650MHz, which means shader clocks around 1500MHz.
As mentioned above, you won't see the card anytime soon, but it's not because it won't be ready but because NVIDIA doesn't have to launch it to stay on top. This is a single core chip, and rumors are that there will be more than plenty of stripped down versions, if needed. It all depends on what AMD/ATI delivers.
We're guessing late Q3 or early Q4, time will tell. And this is not the GeForce 9900 series. The 9900 series will be based on the 55nm G92b core.