I welcome a kicking if I am wrong but I think that the terms have become confused here.
You would think that the X 'server' would be the box running the X apps, wouldn't you? As in "file server". But this is not the case. An X server is a process which 'displays' X programs. Therefore the X server is the machine actually displaying the program.
Therefore the last post would stand to reason now that we have clarifies the terms. Of course you need a graphics card to run an X server, otherwise you wouldn't be able to see anything.
You do not need a graphics card to execute X programs however - so yes, the AIX box does not need a graphics card to execute the code - but the Sun box must in order to run the X server to display the output of the AIX box.
Very confusing!!
I think the confusion comes from applying client-server networking terminology to X. One instantly thinks " I'm connecting to the [AIX] box, therefore it is the X server ".
Hope this is clear(er)