It all depends on how big your scenes are, and how fast is fast enough. If you do large complex scenes, then more GPU memory may be required (though you can always fall back to CPU mode for large scenes). For speed, do you want knock your socks off and impress all your friends (real and on the forums) blow the doors wide open speed (plus break the bank to get it) .... or is really fast good enough?

I attached a performance comparison chart showing the top performers as of about a month ago. If your on a budget. the GTX 960 might be a good option, and you might not need to upgrade your power supply (400W min.). Sure, the 960 isn't nearly as fast as the 980Ti or TitanX - but it is still a great performer (faster than what I'm using and I'm quite pleased with my render times), at a price that really is affordable. With my GTX 960M I get good enough performance that I use the Iray renderer a lot in the viewport when setting up scenes, and always for setting up lighting and shaders. Making sure you convert ALL of the materials in the scene to Iray shaders will speed up the interactive nature of the viewport a lot - waiting for the CPU to convert 3Delight shaders to Iray shaders really slows things down.

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