GTX 580. Still very potent, especially if you want to use it for any video editing or rendering in Blender aswell, for which it's particularly good. 3GB models also exist, which is what I normally hunt for (I've obtained quite a few).
Not sure what the supply is like in the US, they might be a bit more than $50, but worth it for sure. About twice as quick as a GTX 460, or if you prefer, about half the speed of a GTX 970 (and identical to the 970M gfx used in my 17" MSI GE72 6QF Apache Pro gaming laptop).
With all the 970s flooding the market, prices of 580s should be dropping further. The later GTX 760 is faster, but those tend to go for a fair bit more, and are nowhere near as potent for GPU acceleration of apps (the 580 is faster than the entire 600 series for CUDA, and beats all the 700s except the 780 Ti and Titans).
Add a 2nd later for SLI, works nicely. I used two 3GB 580s for a fair while before upgrading to a 980 (the 980 was nicely faster for gaming, but actually about 10% slower for CUDA in Blender, etc.)
Mac070 mentioned the 750 Ti, which is just a little bit slower than a 580, though it does use much less power, etc. However, they sell for a lot more normally, though they certainly are much quieter. Far slower than a 580 for CUDA though of course. I have a 750 Ti in my HTPC.
Ian.
PS. Benchmarks on my site here:
http://www.sgidepot.co.uk/sgi.html#PC
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