Build so far

Here's where I'm at.

Would love to hear what you guys think.

I like it a lot! In my opinion I would replace the GTX 960 with a R9280. same price basically and it has more memory! 

+1 for a 280, 960's 2gb vram will be the kick in the pants. Also I'd personally go with this psu >> - single 12v rail, gold, seasonic oem, full modular.


In terms of CUDA cores and clock speed, which is more important? The GTX 960 has a higher clock speed, but around half the cores

You're comparing two completely different architectures - comparing core count and clock speed is absolutely pointless.

Look up real world benchmarks.  IMO I'd go with an R9 280 as well.