The reason for Windows XP is simple: it supports most higher-end video cards and video software, while newer ones do not. This is because of how Microsoft changed the video/audio subsystems starting with Vista, as well as changed some of the backend functions for display. So, for example, lots of quality Matrox, Canopus, ATI and Aja cards no longer work. And newer cards remove functions, and are tuned to HD work not SD work. You also have capturing, editing, and authoring software that fails to work well, or at all, in post-XP Windows OS.
Yes, ATI AIW is one of the many reasons you'd want an XP system.
non-SP has some issues
SP1 is fine
SP2 is best
SP3 needs all that "security" junk disabled, nag messages especially, as it causes dropped frames when capturing
AGP slot is best, but some PCIe are fine, the ATI AIW x1800 and x1900 being two of them. The ATI AIW 2006 is hit-or-miss for me, and I'm not sure why. For that matter, I remember having issues with an x1900 once as well.
The CPU is nice, and a dual-core or faster allows for 15mbps MPEG-2 capture.
I use something similar, on a backup system: AMD dual-core, Gigabyte board, PCIe ATI AIW, 2gb RAM.
For capturing, the system will never even use 2gb. So don't waste funds there.
Yes, get a quality audio card. The Santa Cruz Turtle Beach is suggested. Never use onboard audio. Be careful that the M-audio board isn't boosting audio levels on you. That's bad for tape capturing. Lots of fancy audio boards are actually terrible due to distortion and drivers.
I edit and restore on my new Skylake, and formerly a Phenom II.