Actually i am curious as well. I currently have 7300GT in my desktop and it performs quite well for Oblivion, Rainbow Six Vegas and STALKER (Low Resolutions). Mind you the 7300GT is a 128-bit bus (8400GS is 64bit) and is based on the 7600 technology with just some disabled shader/vertex pipelines. Today thou its perfromance is borderline for playing the newest games at high details.
I did some tests since i am considering getting a 8400GS laptop. But i think i came to the conclusion that the 8400GS will not reach 7300GT performance. Something i was dissapointed with since the 7300GT performance would have been what i consider ideal for a DX10 mobile thin and light computer.
Also Mobile versions are not the same as their desktop counterparts. A 7400go may not be faster than a 7300 desktop.
3DMARK COMPARISION
-------------------------- 3dmark05 ---- 3dmark06
Nvidia Geforce 6400Go _____ 1019 ________ N/A
Nvidia Geforce 7400Go _____ 1929 ________ 800
Nvidia Geforce 8400M GS ___ 2935 _______ 1456
Nvidia Geforce 7300GT _____ 3555 _______ 1846
http://kettya.com/notebook2/gpu_ranking_mark05.htm
http://kettya.com/notebook2/gpu_ranking_mark06.htm