When you say sloooow, how slow? I'm getting 12 hours for a 1080 HD render on a 2.3 GHz Core i7.
I looked at CUDA core count vs. prices, hoping I'd see one model that stands out as a good value in $$ per CUDA. But it surprised me to see graphics card prices scale almost perfectly linear when it comes to the CUDAs: expect to pay 20 cents per CUDA on most current models, with higher models having the added benefit of more RAM. All of these except the lower 1060 model have 4 GB or more.
TL: DR - buy a gtx 1060 6 gig
one thing to remember is that between "generations" of graphics cards, cuda performance can not be directly compared. 1 cuda core on a 900 is not comparable to 1 on a 1000 series. I'd advise to look at direct benchmark comparisons between your current GPU and the potential purchases. Google is your friend, simply put in your current GPU + "cuda benchmark" and then look for the same benchmark software running on a potential purchase.
i had a 670 gtx mini that i loved, and i thought it was turning out "decent" render time for a bit, and when it died, i picked up a cheap, and yes for 230 bucks on amazon at the time was cheap for a good GPU, 1060 6 gig and i was shocked at the difference in render times and over all performance.
just remember that if you aren't running iray, and instead are running Reality, octane, or luxrender, you can probably run an AMD card on OpenCL, however if you are runnign IRAY, you need Nvidia.
personally, if you aren't running enterprise, missian critical stuff, and you aren't a "gear slut" i'd advise in the 1060 range, they are generally targeted at the "sweet spot" of price/performance ratio, and you'll get decent FPS in games @ 4K, nothing buttery smooth, and at the sub 250 dollar range, if you want to, in the future, it's easy to add a second one in for SLI.
one thing to remember is also "video card memory" can be a cap on your IRAY renders. if the scene you are rendering is larger than your graphics card memory, IRAY defaults to the CPU rendering. so the 6 gig 1060 again is a nice sweet spot, it's not gonna do heavy polygon, heavy texture 4K renders, but it has enough overhead ram wise to get what you need from a hobbyist perspective done.
i hope this helps