I have been curious about using hardware solutions for recording to local files. Do you need to have it running on its own box, with its own CPU, etc? Otherwise I'm guessing you get a hit to your performance.
What if you have high resolution? I often run 3 monitors, at 3840x1024 resolution. Would pushing that many pixels through a hardware cruncher require its own system? How beefy a system do you use for a hardware capture box? Can I purpose an old Core2Duo box for such a thing? Or would that become a bottleneck?
I do use a second box. You do not have to depending on the hardware you use to record with. I would suggest you use a 2nd system though. You will only be able to record a single screen (it may be possible to record all three screens, but the final storage resolution would not be supported, and the left/right views would not be worth viewing separately).
I use the BlackMagic Intensity Pro 4k for recording 1080p60 uncompressed. It will record 2160p30, also. BlackMagic also makes a card for 2160p60, but my Internet upload speed does not support 4k uploads in reasonable time, so I don't use it. The Intensity Pro does require an open PCIe 4x slot, and you may need an HDMI splitter, or mirrored display settings when you record, or both depending on your own desires.
The CPU is not as important as the read/write speed of the second system, or your own single system. I have mine configured as AHCI for OS and RAID for recording, with three drives in RAID0 configuration. I record uncompressed video, so this kind of speed is a requirement. My 2nd box is an LGA1150 with an i7-4770k and 16GB of RAM.
https://www.blackmagicdesign.com/products/intensitypro4khttps://www.blackmagicdesign.com/products/decklink/techspecs/W-DLK-25