Differences between the T400 and the X200 apart from display size?
Are there any other differences than the display size?
T400 has optical drive bay, optional discrete graphics, and optional PCMCIA slot/SD card reader/smart card reader (in addition to the standard Express Card slot, depending on model type).
If you want to buy a T400, those with WXGA+ (1440*900) screen and integrated graphics are recommended. (The optional ATI HD3470 discrete graphics is very weak and useless.)
There is a usable Atheros card for X200, T400 and W500 series. Atheros AR2425 with FRU number 43Y6511.