I can't recommend a specific card, but I have read that the more VRAM it has, the better for HLSL.
I've just ordered a new MAME PC which has a 2GB NVIDIA GeForce GTX 750 Ti, which I'm hoping will be good enough. I'll post back when I get the PC and have given it a test.
Apparently, no on-board GPU is likely to be good enough for smooth HLSL.
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