No offense, or slight, intended towards your dad, but if he REALLY believes that OEM manufacturers use the parts they use together, BECAUSE they are "proven combinations", then he doesn't actually know about computers at all.
The ONLY reason OEM manufacturers use the part combinations they DO use, is because they are generally high enough quality to last for a reasonable warranty period, generally one year (Maybe two, on higher cost systems), and are CHEAP. Profit margins are what drive manufacturers of prebuilt desktop systems, not quality, not performance, not long lifespans. If you want quality parts, with high performance capabilities and product warranties that generally exceed those given for whole system purchases by MANY years especially for things like motherboards, power supplies and graphics cards (Memory usually has a lifetime warranty no matter who you buy it from. Usually), then you buy aftermarket high quality parts that have been shown to be good purchases through professional reviews and testing.
If you want something EASY, that requires little thought, and that you will likely have to throw away as soon as it starts showing signs of failing because most of the parts inside will not be upgradeable due to proprietary considerations, THEN you buy an OEM system.
Obviously, there are some exceptions, such as with enterprise systems and solutions for professional workstations, but on the CONSUMER side of things, that ^^^^is it in a nutshell.