As I already wrote in another forum...
The 9400M appears to be similar to 9100M, but it will have double number of shader units. So, I take that half of the 8600GT performance is realistic, maybe more with shader-intensive (not bandwidth-intensive) applications. I guess most MMOs and older games can be played without any problems, probably even with some AA on. Newer games should also be playable on low settings (I played Crysis with 7900GS and it was, well, ok. I think 9400M won't be slower).
I bet the 9400m will be slower than you 7900GS by miles