In our most recent PC build, we emphasized size and acoustics when choosing components, and it turned out that we didn't have to sacrifice performance to build a powerful PC that didn't look like a giant monolith. The Bitfenix Prodigy case we chose was just the right size to house a GeForce GTX Titan, but we know that many builders using Mini-ITX motherboards won't have cases that are as accommodating. For smaller cases, Asus's new GeForce GTX 670 DirectCU Mini is worth looking at, given its extremely small size.
The card is just 6.7 inches long (17cm), which is a full 4 inches shorter than Asus's previous GTX 670 DC2. For comparison, the Titan we used was 10.5 inches long, and the largest videocard I've ever tested was AMD's 6990 dual-GPU card, which was a full foot long. Asus's new GTX 670 Mini still takes up two PCI slots and requires an eight-pin PCI-E power connector, and is clocked at 928MHz with 2GB of GDDR5 ram. GTX 670 class cards are a good sweet spot for high-end gaming PCs, and this is the smallest I've seen one (previous small GPUs topped out at the 660 Ti, like Zotac's 7-inch ZT-60802).
And if you're looking for a small case for a Mini-ITX build, we've got you covered too.