The low colour depth issue suggest something you installed could have corrupted your graphics card drivers :hmm: a suspect I consider even more justified since your new card -same brand, ATI- suffers the same problem.
Try first of all to re-install directX with latest updates and uninstall & reinstall again your video card drivers.
If you installed some program to watch video that ships its own codecs I would suggest also desinatlling it temporarily while you do your testings.
Re-installing SH3/4 could also help once the above is done, who knows where your system might be storing incorrect instructions for the game engine.
Hope that helps
