The Silverstone SG05 might be a good choice.
Here is a review. If you can live with the limitations of mini ITX motherboard, a 450W PSU good for up to a GTX 680, single slim optical drive, single 2.5" drive (SSD) and single 3.5" HDD, and a graphics card no longer than around 10" and with two 6-pin PCIe power plugs or fewer, then it is about as small as you can get for top notch single GPU performance.