Something weird with Thief 3...

[scionix]

I just bought thief 3, and noticed something very odd. While i run doom 3 at a constant 40-60fps at 1024x768 with 2x AA and high qual. graphics and all options on even in the most heavily lit/shadowed rooms, whenever i hit a heavily shadowed area in thief 3, my computer slows down to a crawl, something in the area of 20 fps, despite seriously toning down image quality (800x600, low-mid qual graphics, low shadow detail, etc.).
This is the system i'm running: Ati 9800Pro 128MB Catalyst 4.7, Athlon 64 3400+, MSI Neo-2, 1024MB 3200 Corsair, 74GB Raptor. My friend runs this game fine with a GeForce 4 and and Athlon XP 2100. Can anyone tell me why this is happening? Thx.