Thief is a good looking game, though perhaps not as good as you might expect. While the rendering technologies we mentioned earlier such as Contact Hardening Shadows and Parallax Occlusion Mapping look nice, Thief's graphics simply aren't on the same level as benchmark titles including Crysis 3 and Battlefield 4.
This is at least partly because those rendering technologies are essentially tacked onto an older game engine (Unreal Engine 3). Even worse news is that because those features aren't natively supported by UE3, they likely come with a bigger performance hit than if the game had been developed with a newer engine.
On the bright side, for now it seems like performance is surprisingly well balanced between AMD and Nvidia GPUs, even though this is an AMD-supported title and Nvidia hasn't released a Thief-oriented driver update. Support for Mantle has also yet to be patched in, so today's performance isn't exactly final.
At the upper end, the R9 290X and GTX Titan delivered nearly the same results while the GTX 780 Ti was only a tad faster. However, Nvidia has an edge in the lower brackets with the GTX 760 and 770 being cheaper and faster than the R9 270X and 280X. Additionally, for the same cash, the GTX 660 outpaced the HD 7850.
Unfortunately, we can't get Crossfire running with the Catalyst 14.2 Beta using HD 7970 or 7990 cards, even though AMD says it should work. After scouring online forums it seems some users have been able to get Crossfire going while many haven't. SLI does seem to work, albeit inefficiently with 40-50% performance gains.
Assuming you're fine with high quality at 1080p or so, you should be able to play Thief with mid-range cards such as the R9 270X and GTX 760, while the sub-$200 GTX 750 Ti offers relatively playable performance at 1680x1050. Maximum quality graphics and higher resolutions will require a flagship GPU, however.