The H50-50 has limited internal space for video cards. It accepts full height half length cards. Which will knock many high end cards out of the running.
The most powerful I know of would be the GeForce GTX 970. However, not all GTX 970 would fit. A reference design card would not fit nor the multi-fan cooler models. Some makers use a more compact design.
Given the low power requirements of the GTX 1070. When it is released on June 10 an OEM may release a compact model. In which case that will be the fastest to fit. Heck a GTX 1080 may even get a compact version due to the lower power requirement thus lower heat output.
If you want to use larger and more powerful cards. You will need to replace the case with a larger ATX Mid or Full tower case which accepts full length cards rather than half length.
Beyond the case limitations. Your power supply (PSU) is limited as well. According to Lenovo PCI-e slot 1 only accepts up to a 75w video card. Which limits you to a low end card like a GeForce GT 740. If you want to use a better card you will need to get a better PSU. I'd suggest one which is at least 650w and 80+ Bronze certified. So you don't have to worry about which card you run.
This is currently the best card I know of which will fit.
http://pcpartpicker.com/part/asus-video-card-gtx970dcmoc4gd5
If you want to see more option. Measure the internal length from back of the case to the drive rack running down the inside of the case. Although it is officially half length you may actually have some more room. Then convert to mm if you are using inches. PCPartpicker.com has length slider which you can set for the maximum card length. Right now the linked slider is set for 173mm to accommodate half length.
http://pcpartpicker.com/parts/video-card/#sort=a3&page=1&l=69,173&c=208,186,185,224,310,311,312&r=24576,16384,12288,8192,6144,4096
These would make for a good case and power supply. If you want to replace the case. Otherwise just get the PSU.
http://pcpartpicker.com/p/tYpfMp
