For Thief and Gothic II there are unofficial graphic mods out there that improve things massively. They basically replace the original models with those from Thief II and Gothic 3, and also fix some bugs.

www.ttlg.com/forums/showthread.php?t=152429 - that’s a user made campaign for Thief, the thread also has links to all the patches and updates. The campaign is also absolutely great with overwhelmingly massive maps, but you should play the original first.