Compared to its 64 bit predecessor, the 8400GS in a huge improvement! About 300 marks more than the 7300GT too

In 3D Mark 06 which is more memory intensive, the 64bit memory path weakness of the 8400GS is more apparent, losing out to the 7300GT.

The 8400GS is only slightly slower than the 8500GT

The crippled 64bit memory bus is apparent in FEAR

Half Life 2 is pretty memory intensive as you can see

 

Quake 4 is pretty memory intensive too.