The PC version of Thief is due its AMD Mantle support update tomorrow from Eidos Montreal, which lets the game use more of the hardware's power bypassing Microsoft's DirectX API should their GPU support it.
So far Mantle has been seen in action with DICE's Battlefield 4, although it's a mixed bag right now. Mantle should theoretically let games enjoy a lot more GPU juice.
You may remember thethat showcases the difference between Mantle and DirectX with its mega-huge space battle, which is available through .
There's also the matter of the TrueAudio update also coming from AMD and Eidos for Thief, which should offload audio processing from the CPU and RAM to the GPU, thus freeing up the CPU to work on other intensive non-audio tasks and provide a stronger frame rate. Sound is rather important in a stealth game like Thief.