A:AnswerHello,
If you are adding it onto the same stick the system comes with, we recommend getting the exact same model so that there are no compatibility issues.
A:AnswerThe $580 pc (991r) has a GeForce GTX 1660, whereas the $800 pc (991) has a Geforce GTX 1660 Super which runs games at a slightly higher fps. As far as I can tell that is the only difference between these two systems since they both have an i5-10400f, 8GB of memory, a 240GB SSD, and a 1TB HDD. If this influences your decision I would recommend the cheaper one as it can save you $120, seeing how the only upgrade you would get with the $800 system is about +10-15 fps with the 1660 super.