Sorry for the personal attack, but hey, you called me an idiot. Granted the reasons I provided were somewhat inflammatory, but with your setup, you should be getting around 35 - 45 fps. Just keep in mind, this isn't a pure speed, twitch game like UT2K4. And for the life of me, I cannot figure out why you think this game looks ugly. Now DX:IW is ****ing ugly, which is why I'm a little confused about all the Thief 3 hate. The boys at Ion Storm did a pretty good job on it. In fact, there's a level in the game called Shalebridge Castle, which ranks among the best levels ever created.
Anyhow, in the default.ini file in your Thief 3 directory, scroll down until you see a section called engine.gameengine. In that section, there should be a line called "cachesizemeg" - it's probably set to 1 - try changing it to 4 or 6; however, somebody actually suggested putting 80% of your ram there, though. Regardless what you do, this will most likely help you out. Also, see the post below.
P.S. Downloading the minimalist HUD patch makes quite a bit of difference as far as framerate. It also reduces the console-sized icon graphics.