I want to buy a Samsung 960 EVO to use with my Haswell MOBO until I switch to an X370 Ryzen board. Do these cards make a difference or should I just buy the cheapest I can get? (with 4x PCiE 3.0 of course)
Any of the cards that are PCIE 3.0 X4 will work identically to the others. Some come with heatsinks, but the 960 evo and 960 pro are really the first M.2 drives to thermally manage themselves well enough to see no improvements if you put a heatsink on them. I'd just go for any cheap one you can find that's good quality. I've used more than a couple. The Angelbird Wings PX2 is a very nice but expensive one, the Asus Hyper M.2 adapters are good as well, but none of them will perform any better than the cheapest one on amazon.
I have a Z-97 mobo paired with an Intel 750 PCIe SSD (windows) and a Samsung 850 EVO (Steam) and a Crucial MX100 (games). I was told (and didn't believe) that unless my use case specifically takes advantage of a super fast SSD (editing big files), I wouldn't be able to tell any difference. They were right.
I am not saying don't get the Samsung 960. Just don't expect a huge difference. My Intel SSD tests really fast as advertised, but in the real world, even when loading a huge game like GTA V, I can't feel that my PCIe SSD is any faster than my SATA 3 SSD's.
Your Mileage May Vary.