I'm an advocate for Soft15KHz too, but they still make PCI cards that could play SF IV at acceptable frame rates...
HIS H545H512P Radeon HD 5450 512MB 64-bit DDR3 PCI Video Card
It's not cutting edge by any means, but it would work.
Also, this is just what Newegg carries. I'd be willing to bet there's even newer chips still being used on PCI cards if you look around a bit.
The biggest limitation with PCI is bandwidth across the bus, though I don't think that would be an issue with typical arcade cabinet resolutions.
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