I've actually already put in an order for the OEM Intel cooler but I think it's the dual-core model, which appears to be aluminum. How much of a difference is it likely to make? I am using a dual-core processor (Core 2 Duo E7600 3.06GHz) and the computer case is pretty well ventilated. I also haven't returned that Cooler Master yet. Would it be worth the time and trouble to try and get the adapter? Or sending these both back and getting the quad-core cooler?

As far as the hard drives, are there any particular specs I need to look for in order to avoid the compatibility issues I've heard about? I think this board only does SATA 1.

I think I might know a way to convert HUFFYUV to a Mac-usable format, but if it doesn't work, I can make converted copies on the capture PC and send them to the Mac, or send the HUFFYUV files to my Lenovo i5 laptop with Windows 10 instead, right?

For capture systems, I use Apevia 450W PSUs, cheap and quality. Too many people overspend of the PSU, and get a heat box for the money -- double punishment.

Hyper 212 is worth the time/funds, OEM coolers are junk.

For AGP AIW, I think you're fine.

Don't forget the capture drives, 2tb, Seagate suggested. SATA2 is fine, it'll just run slower. PCI expansion is also fine, non-VIA chipsets, and runs at SATA1 speed with full SATA2/3 specs. You can capture to those without issues.

The Intel cooler is installed and is loud.

I might return the unused Hyper 212 and get a Noctua NH-D14 or NH D-15, or possibly another one from this list https://www.quietpc.com/lga775-coolers.

Or I could do this:
https://youtu.be/z9WwujbfBX0