A quick reasonably reliable way of telling what type of board you've got is to look at the back.
If the keyboard socket, serial port etc.. is mounted on a little rectangular removable plate then it's ATX.
If they're actually part of the casing then it's quite likely it's an AT board.

The NCR 3272 Minitower is very very likely an ATX based machine

The most cost effective way of capturing digital video is to find a motherboad with built in FireWire/IEE1394 and buy some fast HDs. I've captured video using a Sony VAIO with 350MHz PII before so a processor isn't a major consideration. It's just slooooow when processing it.