Lenovo Thinkcentre E73 graphics card replacement?

How difficult would it be to throw a GTX 960/970 with the Thinkcentre E73? Somewhere I heard it required low profile cards, but if that's the case then I should just be able to put the thing in a larger case.

Yes, you'll need a new case and a larger power supply. If you have the i3 model, use the GTX 960 and a decent quality 450W power supply. If you have the i5 model, you might get by with the GTX 970 and a decent quality 500W power supply.
http://len.factoryoutletstore.com/cat/64272/Lenovo-E73-Small-Tower.html?cid=190818&chid=1600