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I know, but it's the best value for that price today (and good FP32 performance too). But your cards are much better, but not produced anymore and also used not easy to get. I've checked that ;-)

Check out the firepro S9150 for your double precision requirements. These are old enterprise only graphics cards that I have seen slowly coming into circulation for cheap. 2.53 Tflops of FP64, 5.07 Tflops FP32

Yes, that could be an option, it has half the efficiency on FP32, but 4 times higher on FP64. Disadvantage is, that there is no fan and you have to build your own solution for that because it's a server-card. Or am I wrong?

Sorry for taking so long to respond, as it is passively cooled it will require the airflow of the front fans to blow air through, I have been looking to get a few for my server case build, but I am unsure of performance on a standard case.

You raise a very good point though

if you thinking about FP64 maybe you should check out used 7870 they have 0,7 tflops in fp64 and 3tflops in fp32, you can get them for around 100$ or if you want also better performance in fp32 you can check R9 290X they have 5,6tflops in fp32 and 0,7tflops in fp64 for about 150-200$.

They are cheaper to buy, but are used and without warranty. The overall efficiency (FP64/kWh) is equal or less than on current RX Vega 56, and much less on FP32, so that's not an real option.