Sorry to answer you late, sven constable, but strangely this weekend I answered to a similar problem on the softimage mailing list.
Have you considered that the problem may occur because of your GeForce 2 card? Almost every poeple I know, including myself, have problems with their GeForce 2 graphic card.
All of these problems concerns the display. Many times, when performing particular operations (like exiting info-selection), Softimage redraws the content of all viewports, at a killing slow speed.
I'm also on an Athlon system, and sometimes, especially when manipulating actors, my computer simply freezes and I have to reboot.
So, yes there could be some Athlon issues, but I suggest you to check first with another type of card. You see, GeForce is a great choice for gaming and 3ds max. Also, a software developper (Animatek) told me that the OpenGL of GeForce 2 is highly round-corner.
Again, sorry for answering you this late, but I found your post by pure hasard.
Bernard Lebel