Nvidia 1080 is a good one. Take a look at this write up. http://timdettmers.com/2014/08/14/which-gpu-for-deep-learnin... its updated in July 2016 with info about the later versions.

If you're getting a single video card, or one for calculation and one just to do video, make sure motherboard has 16 PCI-e rel3 lanes into the primary card, people are using z170 and z97. For 4 video cards, you want x99 mobo, newegg does a good job of standardizing different mfrs' specs e.g. http://www.newegg.com/Product/Product.aspx?Item=N82E16813132...

Other concerns: you want a full size cage, with the SSD drives all the way at the top, out of the way of the video cards (the largest Nvidia OEM 3 fan cards are almost 13" long, so there's going to be some Tetris if you want a lot of drives and video cards in there). Cooling critical, stuff the box with RAM and Xeon's or i7s.

This is a good tip: older Xeon servers, dual socket, can support good PCI bandwidth with 2 CPUs: https://news.ycombinator.com/item?id=12606481

I've been curious about this too. AWS does have K80 instances available for $0.90/hour which isn't too bad for playing around and as long as they keep updating their infrastructure, you can play with the newest stuff versus having to upgrade your own all the time.

Here is a good PC build, also some software info: http://deeplearningathome.com/2016/09/Building-PC-For-Deep-L...

This is the setup I recently made and has been working great so far. However, most of it was purchased on black friday - you can probably replace any of these components with comparable parts that are currently on sale.

https://docs.google.com/spreadsheets/d/1GgTpPbLdRDvw3Mf_d1W0...

And for another naive question: Why kind of practical/"boring" things can be done with this kind of hardware?

I'm in Latin America, and wonder which kind of niche could be satisfied with this, for more "normal" kind of customers.