When I started with UHD, I had a gtx 780 with a mid tower, it took everything my computer had to run my games smoothly, even then my computer was heating up up to 80C+ degrees, with only 3GB of VRAM, I was lagging when there were too much graphical details on my screen. So that was pretty bad. When you are ready for 4k, don't get any cheap stuff, especially PSU, don't underestimate it, or it'll kill your computer like it did to mine. While I was uograding, I got a 1200w psu, it was defective without protection, so then it causes a power surge and killed over $1000 off of my $1500 computer, so that really sucked, make sure you get a PSU with protection
Edit: I bought my gtx 780 for $510, that power surge stripped it down to $70, my RAM, motherboard was damaged, plus my cpu. My HDD died after a few weeks, lost all data.